brake fluids
Brake fluid is one of the most important operating fluids in a car, the quality of which determines the reliability of the brake system and safety. Its main function is to transfer energy from the brake master to the wheel cylinders, which press the brake linings against the brake discs or drums. Brake fluids consist of a base (its share is 93-98%) and various additives, additives, sometimes dyes (the remaining 7-2%). According to their composition, they are divided into mineral (castor), glycol and silicone.
Mineral (castor)- which are various mixtures of castor oil and alcohol, for example, butyl (BSC) or amyl alcohol (ASA) have relatively low viscosity-temperature properties, as they solidify at a temperature of -30 ... -40 degrees and boil at a temperature of +115 degrees.
Such liquids have good lubricating and protective properties, are non-hygroscopic, and are not aggressive to paintwork.
But they do not meet international standards, have a low boiling point (they cannot be used on machines with disc brakes) and become too viscous already at minus 20 ° C.
Mineral fluids must not be mixed with fluids on a different basis, as swelling of rubber cuffs, components, hydraulic drives and the formation of castor oil clots are possible.
Glycolic brake fluids consisting of an alcohol-glycol mixture, multifunctional additives and a small amount of water. They have a high boiling point, good viscosity and satisfactory lubricity.
The main disadvantage of glycol fluids is hygroscopicity (the tendency to absorb water from the atmosphere). The more water dissolved in the brake fluid, the lower its boiling point, the greater the viscosity at low temperatures, the worse the lubricity of parts and the stronger the corrosion of metals.
Domestic brake fluid "Neva" has a boiling point of at least +195 degrees and is painted light yellow.
Hydrobrake fluids "Tom" and "Rosa" similar in properties and color to "Neva", but have higher boiling points. For the Tom liquid, this temperature is +207 degrees, and for the Rosa liquid, it is +260 degrees. Taking into account hygroscopicity at a moisture content of 3.5%, the actual boiling points for these liquids are +151 and +193 degrees, respectively, which exceeds the same indicator (+145) for the Neva liquid.
In Russia, there is no single state or industry standard that regulates the quality indicators of brake fluids. All domestic manufacturers of TJ work according to their own specifications, focusing on the standards adopted in the United States and Western Europe. (Standards SAE J1703 (SAE - Society of Automotive Engineers (USA), ISO (DIN) 4925 (ISO (DIN) - International Organization for Standardization and FMVSS No. 116 (FMVSS - US Federal Motor Vehicle Safety Standard).
The most popular at the moment are domestic and imported glycol fluids, classified by boiling point and viscosity in accordance with DOT - Department of Transportation (Department of Transportation, USA).
Distinguish between the boiling point of a "dry" liquid (containing no water) and humidified (with a water content of 3.5%). Viscosity is determined at two temperatures: +100°C and -40°C.
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▪ DOT 3 - for relatively slow vehicles with drum brakes or disc front brakes;
▪ DOT 4 - on modern high-speed vehicles with predominantly disc brakes on all wheels;
▪ DOT 5.1 - on road sports cars, where the thermal load on the brakes is much higher.
*Mixing glycol-based brake fluids is possible, but is not recommended as it may degrade fluid performance.
* On vehicles more than twenty years old, the seal rubber may not be compatible with glycol fluids - only mineral brake fluids should be used.
Silicone are made on the basis of organosilicon polymer products. Their viscosity depends little on temperature, they are inert to various materials, workable in the temperature range from –100 to +350°C and do not adsorb moisture. But their use is limited by insufficient lubricating properties.
Silicone-based fluids are incompatible with others.
DOT 5 silicone fluids should be distinguished from DOT 5.1 polyglycol fluids as similar names can lead to confusion.
For this, the packaging additionally indicates:
▪ DOT 5 - SBBF ("silicon based brake fluids" - brake fluid based on silicone).
▪ DOT 5.1 - NSBBF ("non silicon based brake fluids").
DOT 5 class fluids are practically not used in conventional vehicles.
In addition to the main indicators - in terms of boiling point and viscosity, brake fluids must meet other requirements.
Impact on rubber parts. Rubber cuffs are installed between the cylinders and pistons of the hydraulic drive of the brakes. The tightness of these joints increases if, under the influence of brake fluid, the rubber expands in volume (for imported materials, an expansion of no more than 10% is allowed). During operation, the seals should not swell excessively, shrink, lose elasticity and strength.
Impact on metals. Brake hydraulic drive units are made of various metals interconnected, which creates conditions for the development of electrochemical corrosion. To prevent it, corrosion inhibitors are added to brake fluids to protect parts made of steel, cast iron, aluminum, brass and copper.
Lubricating properties. The lubricating properties of the brake fluid determine the wear of the working surfaces of brake cylinders, pistons and lip seals.
thermal stability Brake fluids in the temperature range from minus 40 to plus 100°C must retain their original properties (within certain limits), resist oxidation, delamination, and the formation of sediments and deposits.
Hygroscopicity The tendency of polyglycol-based brake fluids to absorb water from the environment. The more water dissolved in TF, the lower its boiling point, TF boils earlier, thickens more at low temperatures, lubricates parts worse, and the metals in it corrode faster.
On modern cars, due to a number of advantages, glycol brake fluids are mainly used. Unfortunately, in a year they can “absorb” up to 2-3% of moisture and they need to be replaced periodically, without waiting for the condition to approach a dangerous limit. The replacement interval is indicated in the car's operating instructions and usually ranges from 1 to 3 years or 30-40 thousand km.
An objective assessment of the properties of the brake fluid is possible only as a result of laboratory tests. In practice, the state of the brake fluid is assessed visually - in appearance. It should be transparent, homogeneous, without sediment. There are devices for determining the state of the brake fluid by boiling point or degree of moisture. Adding fresh brake fluid when bleeding the system after repair work does little to improve the situation, since a significant part of its volume does not change.
The fluid in the hydraulic system must be completely replaced.
It is necessary to store any brake fluid only in a hermetically sealed container so that it does not come into contact with air, does not oxidize, does not absorb moisture and does not evaporate, in this case the fluid is stored for up to 5 years.
Brake fluid is a very important component of any car. From the name it is clear that it is poured into the brake system and directly contributes to braking, that is, the system works on the hydraulic principle. But pour into the system, do not understand that it is impossible! There are many reasons for this; special compounds with certain properties are used here. Today I want to tell you - what they consist of and why they definitely need to be changed ...
By the way, I received such questions on my blog - “Tell me, is it possible to fill ordinary water into the brake system? And that will be?" It can be seen a young inquisitive mind, as they say - comprehends the world! Read on and you'll understand.
A few words about the brake system
I just want to remind you how it works. In any car there is a brake pedal, if you "roughly exaggerate" it is connected with the brake working cylinder. After you press this pedal, pressure is created in the brake system, it presses on special pistons in the brake calipers (or rear cylinders) that compress (in the case of front-wheel drive) or spread (in the case of rear-wheel drive) brake pads. And already they, in turn, squeeze the brake disc or stop the drum from the inside, I advise you to read -.
I think everyone knows the principle of operation of this system, in our article it is necessary for the next understanding of the material.
System warm-up
When braking, the discs or drums get very hot. Actually, this is a law of physics, friction occurs - brake pads rub against a metal surface, there is a large release of heat. The pads are made of a special “thermo” and “wear-resistant” material, so they can walk for a very long time, this material does not heat up as much as compared to discs or drums.
But their heating can be simply catastrophic (especially near the front axle) there are a lot of videos on the Internet where they heat up to “red”.
And what happens - parts of the brake system, namely pistons, cylinders, and experience huge temperature loads. At high speeds, temperatures can reach up to 150 degrees Celsius. Do you understand what I mean? Actually now we will start talking about the compositions.
What can and cannot be uploaded
Now I want to answer the reader - let's take a look at ordinary water. Why is it STRICTLY IMPOSSIBLE to FILL. YES, actually, everything is simple - even if you don’t dig deep, water boils and evaporates at high temperatures, and at low temperatures it freezes already at -1 degree. You come in this winter, start the car, but there are no brakes! The system is frozen! Also imagine - the system boils when braking, steam pours out of the system tank, the efficiency is at zero. However, even if freezing and boiling are hypothetically defeated, then water has ONE BIG DEFECT - it oxidizes and provokes rust, after a short period of time, the pistons in the calipers will simply rust, the seals that are designed to walk on a smooth and clean surface will break and water will flow out.
Alcohols (in pure form) are also not suitable, because they boil, and even ignite.
Transmission oils, motor oils - oils may be suitable, but again, not all.
They have really low evaporation, they also resist high temperatures very well (especially motor ones), but at low temperatures they can thicken (this negatively affects fluidity), and they can also negatively affect the rubber of the oil seals! That is, calipers or cylinders can leak.
Different formulations of brake fluid
So what did we understand - that the brake fluid must have high fluidity, lubricate, protect against corrosion, not freeze, withstand temperatures in the range of 150 - 170 degrees Celsius, not boil! THAT is such - "super liquid".
Probably, I won’t surprise anyone by saying that “brakes” are still evolving - WELL, NOW THERE IS NO IDEAL COMPOSITION WHICH WOULD 100% MEETING ALL REQUIREMENTS.
Mineral compositions - it all started with them, I will say that they were used on old cars that did not even have front disc brakes, only drums. YES, and speeds at that time rarely exceeded 60 km / h.
Their composition has long been known - castor oil with the addition of butyl or ethyl alcohol, this is a kind of base, but many manufacturers mixed other minerals and petroleum products of various degrees of purification into the composition. It is difficult to call such a composition ideal, but there are still positive points:
- They lubricate well.
- They practically do not absorb moisture, that is, if we say “scientifically”, they have low hygroscopicity
However, there are more cons:
- At temperatures of 110 - 130 degrees boil
- At -20 degrees they begin to thicken
- In addition, castor oil adversely affects parts made of brass, aluminum, copper
- Also, for a long time they could not find a formula that, over time, would not decompose rubber products - oil seals, cuffs, etc.
For a very long time they fought over the formula with castor oil, added all kinds of additives and other substances, but its time has already passed.
Glycol brake fluids - are now used quite widely, you can know under the abbreviations (DOT3, DOT4, DOT 5.1). They contain polyethylene glycols and polyesters of boric acid, comply with all international standards, and have also passed the Russian GOST certification.
This composition is almost perfect, boils at + 150, + 200 degrees, perfectly lubricates, protects against rust, is almost neutral to rubber elements.
The minus here is one and quite large - high hygroscopicity, they absorb moisture very strongly, so it is MANDATORY to change it every 2-3 years! Otherwise, the calipers begin to turn sour and rust.
Silicone brake fluids (DOT5 and special version DOT-5.1/ABS). The composition here is completely different from the counterparts, based on organosilicon polymers. There are enough pluses - it does not absorb moisture, it is absolutely neutral to rubber and metals, it is always fluid (it does not depend on temperature).
There are also disadvantages, and where without them - the lubricating properties are at a low level, so there is more wear on the seals (when compared with counterparts). Such compositions are rarely used on production versions of cars; as a rule, they are poured into sports or racing cars, where the heating of the calipers is much higher.
Reliable operation of the brake system is certainly important for the safety of driving a car, therefore, special requirements are placed on the quality and suitability of the brake fluid. But even if it is of high quality and is chosen correctly, over time its properties will deteriorate during operation, so it is imperative to follow the correct replacement frequency provided by the manufacturer.
When the brake pedal is depressed, the force is transferred hydraulically to the wheel brakes, which slow the vehicle down due to frictional forces. If at the same time the brake fluid can heat up above the permissible limit, boil and form vapor locks. The mixture of liquid and vapor will compress, so the brake pedal may “fall through” and braking will be unreliable, failures may occur. To eliminate this phenomenon in hydraulic drives, special fluids are used for hydraulic drives of the brake system. They are classified by boiling point and viscosity according to DOT (Department of Transportation) standards adopted by the US Department of Transportation. This takes into account the boiling point of a liquid without impurities of moisture (dry), and containing up to 3.5% water. Viscosities - two indicators at a temperature of +100°C and -40°C. Similar requirements are imposed by other international and national standards - ISO 4925, SAE J1703 and others. In Russia, there is no single standard regulating the quality indicators of brake fluids, so manufacturers work according to their own specifications.
What is the composition of brake fluids?
The usual composition is a mixture of a low viscosity solvent (eg alcohol) and a viscous non-volatile substance (eg glycerin).
DOT 3, DOT 4 and DOT 5.1 are based on polyethylene glycol.
DOT 5 is based on silicone, an organosilicon polymer product.
DOT 5.1/ABS is a silicone base with added glycols, especially for vehicles with anti-lock braking system (ABS).
DOT 3, DOT 4 and DOT 5.1 are hygroscopic and absorb moisture from the environment at a rate of about 2-3% per year, while their characteristics vary greatly.
Water absorption worsens the performance of the liquid and sharply reduces the boiling point, at 3.5% water content, the temperature drops from 260 to 140-150 ° C (this is one of the reasons that require regular replacement of the TJ), besides, water causes corrosion, for example, scale forms on the seals, the brake cylinder begins to leak, and it is absorbed so strongly that it is almost impossible to remove it.
DOT 5 is hydrophobic, meaning it does not absorb moisture from the atmosphere, so its service intervals are two to three times longer.
Some manufacturers allow the use of specially designed mineral fluids for specific brake systems on their cars. Mineral TJs are usually produced on the basis of castor oil with the addition of butyl or ethyl alcohol. They have excellent lubricating properties and low hygroscopicity, but a too low boiling point, and they freeze already at a temperature of -20 °. In addition, "mineral water" gradually destroys parts made of copper, brass, aluminum and rubber cuffs of the hydraulic drive. Unlike DOT, mineral oil-based brake fluids are not subject to certification, but rather "cocktails" from different manufacturers who keep their components secret.
Does the fluid change during operation?
Many drivers are in no hurry to change the brake fluid (TF) in their car due to the well-established opinion that it does not change its properties. Such a statement is erroneous, since the brake circuit is considered conditionally closed. The system is equipped with compensation holes, which, when the brake pedal is operated, let air in and out.
During operation, TJ inevitably draws moisture from the air, inevitably changing its composition. One of the undesirable properties of TJ is manifested - hygroscopicity. The fluid needs to be changed.
What is the best liquid to choose?
When choosing a fluid for your car, you should first of all take into account the recommendations of the automaker. For each model of a car of a certain brand, the manufacturer sets the appropriate type of engine, transmission oil, and also recommends the most suitable brake fluid for use. That's why you can't walk into a store and buy the first type of brake fluid you see, even if it's heavily advertised on television and in the press, and praised by salespeople.
When buying brake fluid, be sure to read the directions on the packaging.
The best data contains TJ with a DOT 4 class 6 badge. Many automakers especially recommend the Castrol or Mobil brand and their advice should not be ignored. Of course, you can try and save on the purchase, but we must not forget that high-quality brake fluid can work effectively in the most unforeseen situation, and in addition, it will greatly extend the life of the car's brake system.
Can brake fluids be mixed?
Remember, when buying a certain brand, it is not recommended to mix it with other brands, even if the class and manufacturer are the same. With such mixing, an uncontrolled chemical reaction is formed, which can destroy the elements of the hydraulic system.
Basic properties of brake fluids.
Boiling temperature. The higher it is, the less likely it is that a vapor lock will form in the system. When the car brakes, the working cylinders and the fluid in them heat up. If the temperature exceeds the allowable temperature, the TJ will boil and vapor bubbles will form. The incompressible fluid will become “soft”, the pedal will “fall through”, and the car will not stop in time. The faster the car was driving, the more heat generated during braking. And the more intense the deceleration, the less time will be left for cooling the wheel cylinders and supply pipes. This is typical for frequent prolonged braking, for example, in mountainous areas and even on a flat highway loaded with vehicles, with a sharp “sporty” driving style. The sudden boiling of TJ is insidious in that the driver cannot predict this moment.
Viscosity characterizes the ability of the liquid to pump through the system. The temperature of the environment and the TJ itself can be from minus 40°C in winter in an unheated garage (or on the street) to 100°C in the summer in the engine compartment (in the main cylinder and its tank), and even up to 200°C with intensive deceleration of the car ( in working cylinders). Under these conditions, the change in the viscosity of the liquid must correspond to the flow sections and gaps in the parts and assemblies of the hydraulic system, specified by the vehicle developers. Frozen (all or in some places) TJ can block the operation of the system, thick - it will be difficult to pump through it, increasing the brake response time. And too liquid - increases the likelihood of leaks. 
Impact on rubber parts. Seals should not swell in the TJ, reduce their size (shrink), lose elasticity and strength more than is permissible. Swollen cuffs make it difficult for the pistons to move back in the cylinders, so the car may slow down. With sagging seals, the system will be leaky due to leaks, and deceleration will be ineffective (when you press the pedal, fluid flows inside the master cylinder without transferring force to the brake pads).
Impact on metals. Parts made of steel, cast iron and aluminum should not corrode in the TJ. Otherwise, the pistons will “sour” or the cuffs working on the damaged surface will quickly wear out, and the liquid will flow out of the cylinders or will be pumped inside them. In any case, the hydraulic drive stops working.
Lubricating properties. In order for the cylinders, pistons and cuffs of the system to wear out less, brake fluid must lubricate their working surfaces. Scratches on the cylinder mirror provoke TJ leaks.
Stability- resistance to high temperatures and oxidation by atmospheric oxygen, which occurs faster in a heated liquid. TJ oxidation products corrode metals.
Hygroscopicity The tendency of polyglycol-based brake fluids to absorb water from the atmosphere. In operation - mainly through the compensation hole in the tank lid. Brake fluid has one unpleasant property: it absorbs moisture. Due to constant temperature changes, condensate forms and accumulates in it. The more water is dissolved in TF, the earlier it boils, thickens more at low temperatures, lubricates parts worse, and the metals in it corrode faster. The presence of only 2-3 percent water in the brake fluid reduces its boiling point by about 70 degrees. In practice, this means that when braking, DOT-4, for example, will boil without warming up to 160 degrees, while in a “dry” (that is, without moisture) state, this will happen at 230 degrees. The consequences will be the same as if air has entered the brake system: the pedal becomes a stake, the braking force is sharply weakened.
Features of the operation of brake fluids
The absorption of water from the atmosphere is characteristic of polyglycol-based TF. At the same time, their boiling point decreases. FM VSS normalizes it for “dry”, not yet collected moisture, and moistened, containing 3.5% water, liquids - i.e. limits only limit values. The intensity of the absorption process is not regulated. TJ can be saturated with moisture at first actively, and then more slowly. Or vice versa. But even if the boiling point values of “dry” liquids of different classes are made close, for example, to DOT 5, when they are moistened, this parameter will return to the level characteristic of each class. TJ must be replaced periodically, without waiting for its condition to approach a dangerous limit. The service life of the fluid is assigned by the car factory, having checked its characteristics in relation to the features of the hydraulic systems of its machines.
Checking Fluid Condition
It is possible to objectively determine the main parameters of TJ only in the laboratory. In operation - only indirectly and not all. Independently, the liquid is checked visually - in appearance. It should be transparent, homogeneous, without sediment. In addition, in car services (mainly large, well-equipped, servicing foreign cars), its boiling point is assessed with special indicators. Since the liquid does not circulate in the system, its properties may be different in the tank (test point) and in the wheel cylinders. In the tank, it comes into contact with the atmosphere, gaining moisture, but not in the brake mechanisms. But there the liquid often and strongly heats up, and its stability deteriorates. However, even such approximate checks should not be neglected, there are no other operational methods of control.
Compatibility and replacement
TJ with different bases are incompatible with each other, they delaminate, sometimes a precipitate appears. The parameters of this mixture will be lower than that of any of the original fluids, and its effect on rubber parts is unpredictable. The manufacturer, as a rule, indicates the basis of the TJ on the packaging. Russian RosDOT, Neva, Tom, as well as other domestic and imported polyglycol liquids DOT 3, DOT 4 and DOT 5.1, can be mixed in any proportions. TJ class DOT 5 are based on silicone and are incompatible with others. Therefore, the FM VSS 116 standard requires that "silicone" liquids be colored deep red. The rest of modern TJs are usually yellow (shades from light yellow to light brown). For additional verification, you can mix the liquids in a 1:1 ratio in a glass container. If the mixture is clear and there is no sediment, the TAs are compatible. It should be remembered that it is not recommended to mix liquids of different classes and manufacturers, since their properties may change. Do not mix glycol fluids with castor fluids. The addition of fresh fluid when pumping the system after repair does not restore the properties of the TJ, since almost half of it remains practically unchanged. Therefore, within the time limits established by the car factory, the fluid in the hydraulic system must be completely replaced. 
What are the characteristics of glycol based fluids?
- Half the compression even when heated, resulting in higher system performance and enhanced brake pedal feel.
- water content increases viscosity at low temperatures and enhances corrosiveness;
- corrode paint and irritate the skin;
- the shelf life is very limited due to hygroscopic properties and usually does not exceed 12 months. after opening the container;
- fully compatible with each other (3, 4 and 5.1);
— Easily washed off and neutralized with water.
DOT 5 - how is it different?
- this silicone fluid is absolutely incompatible with glycol;
- has hydrophobic properties, which increases the shelf life (hypothetically to unlimited in sealed containers and 10-15 years after opening) and operation up to 4-5 years;
- since it does not absorb water, any moisture in the system will collect in one place. This can lead to corrosion of the hydraulics. Careful bleeding is necessary to remove all air;
- non-aggressive in relation to paint and varnish coatings;
- has high operating temperatures with an initial boiling point of +260 ° C, designed for use in systems with heavy loads or in extreme conditions, for fast, aggressive driving with frequent and sharp braking. Mainly for vehicles with complex and multi-caliper brake systems;
- a little compression and gives a barely noticeable feeling of "soft pedal";
— prohibited for use in cars with anti-lock braking system (ABS);
- friendly with any rubber parts (complaints that DOT 5 leads to failure of the rubber parts of the brakes were when using early formulas of silicone fluid. Recent compositions have eliminated this problem).
Foreign fluid samples
Specialized car service centers provide diagnostics using test equipment. Experienced drivers often determine "by eye" - by the color of the liquid or the elasticity of the pedal, but it would be more correct to simply observe the replacement terms - in accordance with the recommendations of the car manufacturer and taking into account operating conditions and climate. The universal replacement period for any glycol-based brake fluid is once every two years or after 40 thousand km. run. If the climate is very hot or extreme driving with hard braking is your daily routine, then you will have to change the brake fluid more often, perhaps once a year. Silicone DOT 5 is allowed to be changed every 5 years (but remember, if you have a regular car, forget about silicone). To check the condition of the TJ, there are special devices. Evaluation criterion: if the water in the liquid is less than 3.5%, then it is still suitable, if more, it urgently needs to be changed.
How to replace or replenish fluid?
For refilling, any commercial brand can be used - subject to compliance with the rules and technical requirements. The basic principle is that a fluid can only be replaced by a brand with a higher DOT rating number (for example, DOT 3 can be replaced by DOT 4, and DOT 4 can be replaced by DOT 5.1). And in no case vice versa, the properties of the liquid can change unpredictably.
For a vehicle system filled with DOT 5, none of the other types of brake fluid, i.e. DOT 3, DOT 4 or DOT 5.1 will not work.
Also, mineral and glycol fluids do not combine with each other, if they are mixed, the rubber cuffs of the hydraulic drive are deformed.
Brake fluid is the most important consumable component in a car system. For what purposes does brake fluid serve, when to replace it and which fluid is better to use, read the article.
Appointment of brake fluids
Transfer force from the master brake cylinder to the wheel cylinders. The task, although narrow, is extremely responsible; the brake system has no right to fail under any circumstances. When fluid does not leak in the hydraulic brake drive, it would seem that no attention should be paid to it. However, the efficiency of braking and the stability of the system depends on its condition. If, for example, bad antifreeze or engine oil only shortens the life of the engine, then poor quality brake fluid can lead to an accident.
Brake fluid (TF) consists of a base (its share is 93-98%) and various additives (the remaining 7-2%). Outdated fluids, such as "BSK", are made from a mixture of castor oil and butyl alcohol in a 1: 1 ratio. The basis of modern, the most common, including ("Neva", "Tom" and RosDOT, aka "Rosa"), are polyglycols and their ethers. Silicones are used much less often. In the complex of additives, some of them prevent the oxidation of fuel oil by atmospheric oxygen and during strong heating, while others protect the metal parts of hydraulic systems from corrosion. The basic properties of any brake fluid depend on the combination of its components.
Boiling temperature. The higher it is, the less likely it is that a vapor lock will form in the system. When the car brakes, the working cylinders and the fluid in them heat up. If the temperature exceeds the allowable temperature, the TJ will boil and vapor bubbles will form. The incompressible fluid will become “soft”, the pedal will “fall through”, and the car will not stop in time. The faster the car was driving, the more heat generated during braking. And the more intense the deceleration, the less time will be left for cooling the wheel cylinders and supply pipes. This is typical for frequent prolonged braking, for example, in mountainous areas and even on a flat highway loaded with vehicles, with a sharp “sporty” driving style. The sudden boiling of TJ is insidious in that the driver cannot predict this moment. 
Viscosity characterizes the ability of the liquid to pump through the system. The temperature of the environment and the TJ itself can be from minus 40°C in winter in an unheated garage (or on the street) to 100°C in the summer in the engine compartment (in the main cylinder and its tank), and even up to 200°C with intensive deceleration of the car ( in working cylinders). Under these conditions, the change in the viscosity of the liquid must correspond to the flow sections and gaps in the parts and assemblies of the hydraulic system, specified by the vehicle developers. Frozen (all or in some places) TJ can block the operation of the system, thick - it will be difficult to pump through it, increasing the brake response time. And too liquid - increases the likelihood of leaks.
Impact on rubber parts. Seals should not swell in the TJ, reduce their size (shrink), lose elasticity and strength more than is permissible. Swollen cuffs make it difficult for the pistons to move back in the cylinders, so the car may slow down. With loose seals, the system will be leaky due to leaks, and deceleration will be ineffective (when you press the pedal, the fluid flows inside the master cylinder, not transmitting force to the brake pads).
Impact on metals. Parts made of steel, cast iron and aluminum should not corrode in the TJ. Otherwise, the pistons will “sour” or the cuffs working on the damaged surface will quickly wear out, and the liquid will flow out of the cylinders or will be pumped inside them. In any case, the hydraulic drive stops working.
Lubricating properties. In order for the cylinders, pistons and cuffs of the system to wear out less, brake fluid must lubricate their working surfaces. Scratches on the cylinder mirror provoke TJ leaks.
Stability- resistance to high temperatures and oxidation by atmospheric oxygen, which occurs faster in a heated liquid. TJ oxidation products corrode metals.
Hygroscopicity– the tendency of polyglycol-based brake fluids to absorb water from the atmosphere. In operation - mainly through the compensation hole in the tank lid. Brake fluid has one unpleasant property: it absorbs moisture. Due to constant temperature changes, condensate forms and accumulates in it. The more water is dissolved in TF, the earlier it boils, thickens more at low temperatures, lubricates parts worse, and the metals in it corrode faster. The presence of only 2-3 percent water in the brake fluid reduces its boiling point by about 70 degrees. In practice, this means that when braking, DOT-4, for example, will boil without warming up to 160 degrees, while in a “dry” (that is, without moisture) state, this will happen at 230 degrees. The consequences will be the same as if air has entered the brake system: the pedal becomes a stake, the braking force is sharply weakened.
Brake Fluid Classes
When developing fluids, as a rule, they are guided by the requirements of the American FMVSS No. 116 (DOT) vehicle safety system. Liquids are classified by boiling point and viscosity (see table), their other properties are similar.
Which TJ should be used in a car is decided by its manufacturer. The brake system of a car (including rubber and structural materials) is developed for a certain type of brake fluids, so domestic fluids should not be used on foreign cars - and not because ours are worse, but imported ones are better. It's just that each machine is made of its own materials, and different TJs can affect them differently. The main rule for using brake fluid is to follow the instructions that came with the car.
DOT 3 fluids are designed for hydraulic drum brakes, as well as for disc brakes under normal operating conditions. DOT 4 fluids are used on vehicles with disc brakes operating in urban areas (in “acceleration-braking” modes). Alcohol-castor liquid "BSK" cannot be considered as a TJ for modern cars. It was developed for old cars of the GAZ-21 era and freezes already at a temperature of -20 ° C. The “Neva” liquid of brand “A” is slightly inferior to the requirements of DOT 3, and brand “B” does not meet them in terms of boiling point, both dry and and moistened fluid. TJ "Neva" was developed for use in the braking systems of the first models of "Zhiguli". Brake fluids DOT 3, "Tom" and DOT 4 can be used on almost all domestic cars. 
DOT5 brake fluid is also known as “silicone” brake fluid. Its advantages: does not corrode paint; does not absorb water and may be useful where absorption is a problem; is compatible with any rubber parts. Disadvantages: DOT5 cannot be mixed with DOT3 or DOT4. Most problems with DOT5 are likely due to mixing with some other type of brake fluid. The best way to upgrade to DOT5 is to completely overhaul the hydraulic system. Complaints about DOT5 causing brake rubber failure were common in early DOT5 formulations. It was believed that the reason for this was the inappropriate use of various additives. In the latest formulas, this problem has been eliminated. Since DOT5 does not absorb water, any moisture present in the hydraulic system will accumulate in one place. This can cause localized corrosion in the hydraulics. Careful bleeding is necessary to remove all air in the system. Small bubbles may form in the liquid, which increase in size over time. Several pumps may be required. DOT5 is somewhat compressive (which gives a subtle "soft pedal" feel). The boiling point of DOT5 is lower than that of DOT4.
DOT5.1 brake fluid is relatively new, so it constantly misleads motorists. This misconception could have been avoided if this brake fluid had been called differently. The designation “5.1” may suggest that this is a silicone-based modification of DOT 5 brake fluid. It would be more natural to call it 4.1. or 6, since DOT5.1 is glycol based, like DOT3 and DOT4, not silicone based, like DOT5. As for the principle nature of 5.1 brake fluid, it can be defined as a “high-tech” DOT4 brake fluid rather than the traditional DOT5. Benefits: DOT5.1 provides superior performance compared to other brake fluids discussed in this article. It has a higher boiling point than DOT3 or 4, both initial and final. In fact, the end boiling point (about 275 degrees C) is almost the same as racing brake fluids (about 300 degrees C), and the initial boiling point of 5.1 brake fluid (about 175-200 degrees C) is naturally much higher than racing brake fluids. liquids (about 145 degrees). DOT5.1 is considered to be compatible with all rubber components.
Disadvantages: DOT5.1 are not silicone brake fluids, hence they absorb water. DOT5.1, like DOT3 and DOT4, will corrode paint. Silicone-free DOT 5.1 fluids are sometimes referred to as DOT 5.1 NSBBF, and silicone DOT 5 to DOT 5 SBBF. The abbreviation NSBBF stands for “non silicon based brake fluids” and SBBF stands for “silicon based brake fluids”.
Features of the operation of brake fluids
The absorption of water from the atmosphere is characteristic of polyglycol-based TF. At the same time, their boiling point decreases. FM VSS standardizes it for “dry” liquids that have not yet collected moisture, and moistened liquids containing 3.5% water - i.e. limits only limit values. The intensity of the absorption process is not regulated. TJ can be saturated with moisture at first actively, and then more slowly. Or vice versa. But even if the boiling point values of “dry” liquids of different classes are made close, for example, to DOT 5, when they are moistened, this parameter will return to the level characteristic of each class. TJ must be replaced periodically, without waiting for its condition to approach a dangerous limit. The service life of the fluid is assigned by the car factory, having checked its characteristics in relation to the features of the hydraulic systems of its machines.
Checking Fluid Condition
It is possible to objectively determine the main parameters of TJ only in the laboratory. In operation - only indirectly and not all. Independently, the liquid is checked visually - in appearance. It should be transparent, homogeneous, without sediment. In addition, in car services (mainly large, well-equipped, servicing foreign cars), its boiling point is assessed with special indicators. Since the liquid does not circulate in the system, its properties may be different in the tank (test point) and in the wheel cylinders. In the tank, it comes into contact with the atmosphere, gaining moisture, but not in the brake mechanisms. But there the liquid often and strongly heats up, and its stability deteriorates. However, even such approximate checks should not be neglected, there are no other operational methods of control.
Compatibility and replacement
TJ with different bases are incompatible with each other, they delaminate, sometimes a precipitate appears. The parameters of this mixture will be lower than that of any of the original fluids, and its effect on rubber parts is unpredictable. The manufacturer, as a rule, indicates the basis of the TJ on the packaging. Russian RosDOT, Neva, Tom, as well as other domestic and imported polyglycol liquids DOT 3, DOT 4 and DOT 5.1, can be mixed in any proportions. TJ class DOT 5 are based on silicone and are incompatible with others. Therefore, the FM VSS 116 standard requires that "silicone" liquids be colored deep red. The rest of modern TJs are usually yellow (shades from light yellow to light brown). For additional verification, you can mix the liquids in a 1:1 ratio in a glass container. If the mixture is clear and there is no sediment, the TAs are compatible. It should be remembered that it is not recommended to mix liquids of different classes and manufacturers, since their properties may change. Do not mix glycol fluids with castor fluids. The addition of fresh fluid when pumping the system after repair does not restore the properties of the TJ, since almost half of it remains practically unchanged. Therefore, within the time limits established by the car factory, the fluid in the hydraulic system must be completely replaced.
Reliable operation of the brake system is certainly important for the safety of driving a car, therefore, special requirements are placed on the quality and suitability of the brake fluid. But even if it is of high quality and is chosen correctly, over time its properties will deteriorate during operation, so it is imperative to follow the correct replacement frequency provided by the manufacturer.
When the brake pedal is depressed, the force is transferred hydraulically to the wheel brakes, which slow the vehicle down due to frictional forces. If at the same time the brake fluid can heat up above the permissible limit, boil and form vapor locks. The mixture of liquid and vapor will compress, so the brake pedal may “fall through” and braking will be unreliable, failures may occur. To eliminate this phenomenon in hydraulic drives, special fluids are used for hydraulic drives of the brake system. They are classified by boiling point and viscosity according to DOT (Department of Transportation) standards adopted by the US Department of Transportation. This takes into account the boiling point of a liquid without impurities of moisture (dry), and containing up to 3.5% water. Viscosities - two indicators at a temperature of +100°C and -40°C. See the table below for these figures (according to the US Federal Standard). Similar requirements are imposed by other international and national standards - ISO 4925, SAE J1703 and others. In Russia, there is no single standard regulating the quality indicators of brake fluids, so manufacturers work according to their own specifications.
Application of different classes of brake fluids:
DOT 3 - on relatively low-speed vehicles with drum-type brakes or front disc brakes;
- DOT 4 - on modern high-speed vehicles with disc brakes on all wheels;
- DOT 5.1 - on sports cars, with higher thermal loads. Liquids of this class are practically not used on ordinary cars.
PERFORMANCE REQUIREMENTS
In addition to the main indicators, the boiling point and viscosity, other equally important requirements are imposed on brake fluids.
The liquid must not harm the rubber parts of the car.
Between the pistons of the hydraulic drive of the brakes and the cylinders there are rubber cuffs, the tightness of which increases under the influence of brake fluid. At the same time, rubber compounds increase in volume, expansion up to 10% is allowed. They should not swell too much, shrink, lose elasticity and strength.
Brake fluid must protect metals from corrosion.
Metal components of the hydraulic brake drive can be subjected to electrochemical corrosion. To prevent this process, corrosion inhibitors must be added to the brake fluid, designed to protect steel, cast iron, aluminum, brass and copper parts.
Lubrication of moving parts.
Brake fluid must have lubricating properties to reduce wear on the working surfaces of brake cylinders, pistons and lip seals.
Stability at low and high temperatures.
Brake fluids work in the temperature range from -40 to +100°C. Within these temperature limits, the liquid must retain the properties provided by the manufacturer, with some degree of fluctuation, resist oxidative processes, delamination, the formation of deposits and deposits.
TYPES AND COMPATIBILITY OF BRAKE FLUIDS
Brake fluids are mineral, glycol and silicone based (about 93-98%), with various additives, additives, dyes.
Mineral base is a 1:1 mixture of alcohol, such as butyl, and castor oil. Such a liquid has good lubricating and protective properties, is non-hygroscopic, and does not damage the paintwork. But it has significant shortcomings that do not allow it to meet international standards. Mineral-based brake fluid has a low boiling point, should not be used on vehicles with disc brakes, and a too high viscosity already at -20°C.
Do not mix mineral and glycol fluids. This can lead to excessive swelling of the hydraulic rubber seals and the formation of castor oil clots.
Glycol brake fluids- based on polyglycols and their esters - a group of chemical compounds of polyhydric alcohols. They have a high boiling point, good viscosity and good lubricating properties. The main disadvantage is hygroscopicity, i.e. the ability to take moisture from the air through the compensation hole in the reservoir cap of the main brake cylinder. Moisture saturation reduces the boiling point of the glycol fluid, increases viscosity at low temperatures, reduces lubricity and corrosion resistance. All glycol fluids, both imported and domestic, DOT 3, DOT 4 and DOT 5.1 classes are interchangeable, they can be mixed, but this is not recommended. This can lead to deterioration of their basic properties.
On fairly old vehicles, over 20 years old, the rubber seals may not be compatible with glycol. Only mineral brake fluids can be used here, otherwise it will lead to the destruction of the cuffs.
Silicone brake fluids are made on the basis of silicon-organic polymeric products. Main advantages: viscosity practically does not depend on temperature, is inert to various materials, is efficient in the temperature range from -100 to +350°С, moisture is not taken from the air. But with all the pluses, such liquids have weak lubricating properties, which limits their use. Silicone fluids do not mix with others.
DOT 5 silicone based fluids should be distinguished from DOT 5.1 polyglycol fluids, similar names can lead to confusion. Usually on the packaging they additionally indicate:
DOT 5 - SBBF ("silicon based brake fluids" - silicone brake fluid).
DOT 5.1 - NSBBF ("non silicon based brake fluids" - non-silicon).
CHECK AND REPLACE
Most modern vehicles use glycol brake fluids, which have a number of advantages. But, unfortunately, in a year, glycol will take up to 2-3% of moisture from the air, and the fluid has to be changed periodically, and in advance, while it began to pose a danger to the reliable operation of the brake system. (see pic). Replacement periods are usually indicated in the car's operating instructions, which range from 1 to 3 years.
It is possible to give an objective assessment of the properties of the brake fluid only in laboratory conditions, therefore, to save time, the condition of the brake fluid is assessed visually. Its transparency, uniformity, absence of sediment are evaluated. There are also devices for determining the boiling point of the brake fluid and the degree of its moisture.
Since the liquid does not circulate in the system, its condition in the tank (test point) may differ from that in the wheel cylinders. In the tank, it can pick up moisture from the air, but not in the brake mechanisms. But there the liquid heats up more strongly, sometimes excessively, and its properties may deteriorate.
If you simply add new brake fluid when pumping the system after repair work, then this will practically not fix the situation, a significant part of the volume will not change.
The fluid needs to be completely changed. The sequence and features of replacing the brake fluid, for example, when pumping with the engine running, depends on the design of the brake system (type of booster, availability of anti-lock devices, etc.). This information can be found in the vehicle's owner's manual.
On domestically produced vehicles, brake fluid is replaced as follows:
Method 1. The old fluid is completely drained by opening all valves (fittings) for releasing air from the hydraulic brake drive. Then the tank is filled with new fluid and, pressing the brake pedal, it is pumped into the system. In this case, the valves must be sequentially closed when liquid appears from them. Then you need to remove air from each hydraulic circuit (“bleeding” the brakes). When using this method, the new liquid does not mix with the old. Part of the new fluid released during pumping can be used again, after allowing it to settle and filtering.
Note. Before replacement, a drain hose is put on each valve, the other end of which is lowered into a suitable container. In this way, it is possible to prevent damage to tires and paintwork on suspension parts, brakes by leaking brake fluid.
Method 2. Constantly topping up the tank of the main cylinder with fresh liquid, each circuit is pumped in turn, thus displacing the old liquid and preventing the system as a whole from draining. This is done until new fluid appears from the valve. The advantage of this method is that air does not enter the hydraulic drive, which makes control pumping unnecessary. But at the same time, it is not excluded that part of the old liquid will remain in the system. In addition, a larger amount of fresh fluid will be required than with the first method, since most of it removed from the hydraulic drive mixes with the old one and becomes unsuitable for further use.
SAFETY PRECAUTIONS FOR HANDLING BRAKE FLUID
Any brake fluid, regardless of type, is stored only in sealed containers, without contact with air, to prevent its oxidation, moisture accumulation and evaporation.
Remember that brake fluid is usually flammable or flammable. It is strictly forbidden to smoke while working with it. It is poisonous, if ingested, even 100 ml, can be fatal. Brake fluid usually smells like alcohol and can easily be confused with an alcoholic beverage. If you accidentally swallow liquids, for example when pumping out of the reservoir of the master cylinder, you should immediately flush the stomach. In case of contact with eyes, rinse with plenty of water. In any case, in such situations, you should consult a doctor.
