Checking the brakes is an integral part of the technical inspection. If you have any doubts about their serviceability, you need to check them. You need to check the brakes on a flat area, away from fences and other vehicles. It will be unfortunate if your brakes really are faulty, and there is a concrete slab ahead, or worse, someone's Lexus.
It all depends on the brake system installed on your car. There are several options for braking systems. The oldest systems do not have a vacuum booster and ABS systems, and it's very easy to check them. First, let's define the details of such a scheme. It consists only of brake cylinder hoses and pipelines. The brake cylinder under the pedal is called MAIN.
The brake cylinder located at the wheel is called WORKER.
They are connected brake hoses and pipelines. Today, almost all brake systems are dual-circuit. This means that if one cylinder leaks or breaks, only two wheels fail, not the entire brake system. One circuit unites diagonally opposite wheels. So, to check brake system you need to do the following.
- The pedal should not fall through the floor. If you press the pedal several times and nothing changes, then the problem is the lack of tightness of the brake system. Brake fluid is leaking from some cylinder and has most likely already leaked out. You can check this by looking at the expansion tank. In order to fix the problem, you need to find the place of the leak. It is very simple to determine this - the brake fluid will be viscous and oily. You need to look for leaks first of all near the brake cylinders. Leaks from the working cylinders are most often located behind the wheels. If the problem is with the master cylinder and you don’t have a vacuum booster, then you need to look for leaks directly on it or in the car’s interior. Run your fingers under the cylinder and everything will immediately become clear. If brake fluid leaks into the passenger compartment, it can be seen near the brake pedal stem. If a vacuum booster is used, fluid will leak into the booster. In order to check this, you need to separate the brake master cylinder from the vacuum and look. If liquid has leaked into the vacuum, it must be drained. Otherwise, it may corrode the membrane, and the vacuum booster will have to be thrown out. Leaks are most often caused by damage to the cuff, less often by scratches on the mirror. brake cylinder. To fix the problem, you need to replace the cuffs. If this does not help, the cylinder must be replaced.
- If the brake pedal first fell into the floor, and after a few clicks found a stop, then the system is airy. The reasons for the appearance of air can be very different. Most often due to lack of tightness. To replace the leaked fluid comes a new one from expansion tank. After all the liquid has drained from the expansion tank, air enters the system. Air can also enter as a result of boiling brake fluid. It is best to use DOT4, you can interfere with brake fluids but according to the instructions. Getting rid of this air in the system is easy, but painful. This requires two people. The first one must sit driver's seat, the second is to vent the system. As a rule, air appears in one of the circuits. You need to start airing the system from the wheels farthest from the master cylinder. IN brake caliper there is a special bolt for de-airing the system. Similar designs are used in batteries. To de-air them, you need to bleed the air. The bolt in the caliper is steel, while the caliper is usually aluminum or cast iron. As a result of oxidation, these bolts are very difficult to unscrew. There are special keys for this.
The person who is driving must press the brake pedal 9 times and press the pedal 10 times without releasing it. While one holds the pedal, the second unscrews the bolt to release the air. Liquid with air bubbles will flow from under the bolt. After that, the bolt must be tightened.
(It is not worth unscrewing the bolt completely. It is enough to make half a turn and the fluid will run out from under it. Brake fluid - harmful substance, it can not be poured onto the ground, its vapors are also harmful. Therefore, we collect it in a bottle with a hose.)
The pedal will immediately fail at this point. It must not be released until the air bleed screw is tightened. Then the pedal can be released.
Repeat the procedure until the air stops bubbling from under the bolt.
Even if liquid leaves the expansion tank, add it and do not allow the tank to completely empty. Bleeding the system is more difficult than replacing cuffs.
- You need to check the vacuum booster as follows, remember that the pedal should not fail. To get started, you just need to press the brake pedal with the engine off, you need to do this several times. Each time the pedal should get tighter. The brake pedal must not rest against the carpet or the body. After that, you need to press the pedal and start the engine. He must work for Idling and the pedal should become soft. And go a little to the floor. At the same time, the engine speed should not change. If the engine speed drops or the pedal does not become softer, this means that the problem is with the vacuum booster. It is almost impossible to repair it at home. It is better to contact the service station, but it is easier to buy a new one. The cost of repair will be commensurate with the purchase of a new vacuum. All breakdowns of the vacuum booster are associated with a lack of tightness. It can be either a torn membrane or a cracked hose that is connected to the amplifier. characteristic feature lack of tightness is a trimming motor.
- Checking the ABS system is as follows. Need to go to slippery surface, for example, gravel, accelerate the car to a speed of 40 km / h, then sharply apply the brakes. The brake pedal should kick back against the force of your foot. Feels like a ratchet under the pedal. It's a sign working system. The causes of failure are often associated with electronics. On-board computer Very reliable system, the probability of its breakdown is very low, unlike sensors on wheels. They work according to different principles. Starting from coils placed near the gear, ending with laser control, according to the principle computer mouse. Depending on the make and model of your vehicle.
If the brakes disappear while driving and you have some time, you can use hand brake or engine braking. If there is no time, and it is impossible to avoid a collision, look for a softer parapet or choose the lesser of all possible evils.
In the event of failure of the brake cylinders, depending on the type of defect, the rear wheels may be blocked when released or vice versa continue to rotate freely when the brake pedal is pressed. In any case, you should not wait for this moment, especially since it may well be made by yourself.
The principle of operation of the rear brake cylinder
The rear brake cylinders are elements of the brake system that are directly responsible for the impact on the pads, pressing them against the inner surface of the drum.
The main working parts of the rear brake cylinder are the body, two pistons, and two rings - sealing and thrust (adjusting). When you press the brake pedal, the pistons move out of the housing, pressing the pads against the surface of the drum. They return back under the action of the coupling spring of the pads. The task of the thrust ring is to limit this movement, so that the gap between the friction linings and the drum is maintained at the same level. The sealing ring, in turn, prevents leakage of brake fluid through the cavities in which the pistons are located.
1 - stop pads; 2 - protective cap; 3 - cylinder body; 4 - piston; 5 - sealant; 6 - support plate; - 7 - spring; 8 - crackers; 9 - thrust ring; 10 - stop screw; 11 - fitting; A - a slot on the thrust ring
Thus, it is the rear brake cylinder that is responsible for transferring the braking force to the pads and for maintaining the distance between the drum surface and the friction linings as the latter wear. Below you will learn how to check the rear brake cylinder.
Symptoms of the failure of the rear brake cylinder and their check
To check the operation of the rear brake cylinder, you need to identify the symptoms. Symptoms of failure of the rear brake cylinders can be different. Indirect causes for concern may include:
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- a slight but steady drop in the level of brake fluid in the reservoir;
- insufficient braking force on rear axle or uneven braking of the left and right rear wheels.
However, the above may be caused by other problems. Assessing the "well-being" of the cylinders is probably possible only with their visual inspection. This is usually done when replacing the rear brake pads.
Damage to the sealing ring may be evidenced by smudges of brake fluid on their surface. A rather serious cause for concern for the condition of the cylinders may be their overly oxidized surface. IN this case pistons may no longer walk so briskly in the body. You can check this with an assistant who should smoothly press the brake pedal or monitor the piston stroke and hold the pads while you do it. You need to “brake” very carefully, otherwise the pistons may completely come out of the cylinder.
Before checking the effectiveness of the brakes and their adjustment, it is necessary to check the condition and tightening of the fasteners of all components of the brake system, the presence of locking devices (cotter pins, etc.). as well as the general condition of the parts brake mechanism: friction linings(wear, oiling), brake drums(inner surface), return springs of pads and their fastening, fastening brake discs and pads and their free rotation on the axles, etc. After eliminating the defects noticed, the condition and operability of the drive devices are checked.
In a hydraulic drive, check the level of brake fluid in the reservoir of the master brake cylinder. The liquid level should be 10-15 mm below the edge of the filling hole.
Before adding brake fluid to the reservoir of the master brake cylinder, clean the air hole in its plug. If there is air in the brake system, it is pumped. To do this, remove from the right rear wheel bypass valve cap and put on its spherical end a rubber hose 400-500 mm long. The free end of the hose is immersed in a 1/2 liter glass vessel filled with 1/3-1/2 brake fluid. Having unscrewed the bypass valve by 1 / 2-3 / 4 turns, sharply press the brake pedal several times and slowly release it, pumping fluid from the master cylinder into the vessel.
Pumping continues until the release of air bubbles stops from the hose lowered into the jar. While pumping, add liquid to master cylinder without allowing the liquid level to drop by more than half. After that, holding the pedal depressed, wrap the bypass valve. Air is released sequentially from all wheel brake cylinders, starting with the right rear wheel, then from the right front, left front and left rear. On the front brakes, which have two wheel brake cylinders, pumping starts from the lower cylinder.
Rice. Reservoir for filling hydraulic system brake fluid:
1 - pressure gauge; 2 - fitting; 3 - safety air valve; 4 - ball shut-off valve; 5 - hose tip
For cars with a hydrovacuum foot brake booster (GAZ-53A car), it is pumped to remove air from the hydraulic cylinder of the booster. For this purpose, the hydraulic cylinder is provided with relief valves.
Highway hydraulic drive can be filled with brake fluid using a reservoir under pressure of 1.5-2.0 kg/cm2. Fluid is supplied from the tank through a flexible hose through the filler hole of the master brake cylinder. To prevent air from the tank from entering the hydraulic drive system, a ball (or cylindrical) hollow valve 4 with a rubber seat is provided.
The main requirements for the brake fluid: low viscosity within the temperature range from -40 to +40 ° C, enough heat boiling point (+ 115-:- +125°C), as well as low temperature solidification of the order of -40 ° -t 45 ° C and the absence of a destructive effect on the rubber parts of the drive system. Most widespread received brake fluid BSK (TU 1608-47) and ZSK (TU 4226-57). BSC liquid consists of castor oil (47%) and butyl alcohol (53%) with an admixture of an organic dye (red). ESC brake fluid contains castor oil and ethyl alcohol in the same ratio and has a color from orange to red. Mixing of two liquids having a different base (castor oil or glycerin) is not allowed in order to avoid their separation.
In the pneumatic brake drive system, check the air pressure and the tightness of the system and perform the necessary adjustment work. When the compressor unit is in good condition, the increase in pressure in the system from zero to the maximum value (7-8 kg / cm2) when operating at medium speed crankshaft engine should occur within 5-6 minutes. Causes insufficient pressure if there is no air leak in the system, there may be wear piston group compressor and low tension of the compressor drive belt. Fine tensioned belt must bend between the fan and compressor pulleys with a force of 3-4 kg per 10-15 mm. The tightness of the system in the section compressor - brake valve is checked by a pressure gauge at idle engine and the released brake pedal. Pressure (from 7-8 kg/cm2) should not fall faster than 0.1 kg/cm2 in 10-12 minutes.
In the section between the brake valve and the brake chambers, the tightness is checked by the pressure drop when the brake is pressed to failure and the engine is not running. In this case, the pressure should drop sharply by 1-1.5 kg/cm2 and not decrease further.
A continuous decrease in pressure indicates an air leak in the system. Places of air leakage can be determined by ear and by wetting these places with soapy water. The leak is eliminated by replacing parts, tightening and adjusting. The air pressure in the brake system is also checked by connecting a pressure gauge instead of one of the brake chambers. When the engine is idling and the brake pedal is released, the pressure according to the pressure gauge on the instrument panel should be maximum (7-7.35 kg / cm2), and according to the pressure gauge near the brake chamber, equal to zero. On a ZIL-13O car, this check can be performed using one pressure gauge mounted on a dashboard and having two scales: the upper one, showing the pressure in the cylinders, and the lower one, in the brake chambers.
For vehicles that work with trailers and are equipped with an output compressed air to join pneumatic system trailer brakes, check and adjust the air pressure at the outlet using the control pressure gauge.
In the pneumatic drive system, the tightness is checked using a soap emulsion. safety valve and triggering it when it reaches maximum pressure. Adjust the valve if necessary.
Spontaneous braking of the car on the go with the pedal released due to a loose fit inlet valve the control valve is eliminated by cleaning and grinding the valve to the socket, as well as adjusting its position.
The effectiveness of the brakes is checked in one of the following ways:
- on the trail of car braking
- by maximum deceleration
- according to the magnitude of the braking force or static friction torque, measured on each wheel of a stationary vehicle
- by the magnitude of the braking force measured on each wheel, taking into account the manpower of a moving vehicle
In the first method of control, the car on a horizontal dry section of the road (with normal tire pressure) is accelerated to a speed of 30-40 km / h and sharply braked with a foot brake to the “skid”. According to the degree of similarity between the traces left by the wheels on the road and the signs of the car skidding, they judge the simultaneity of the brakes and the uniform distribution of the braking force over the wheels.
At smooth braking from an initial speed of 30 km/h, the braking path of the vehicle must be within the limits set specifications. Although this method of control is widely used in practice, it leads to intensive tire wear and cannot be considered appropriate.
In the second test method, the effectiveness of the brakes is evaluated by the maximum deceleration determined by the decelerometer.
The pendulum-type decelerometer consists of a molded body (made of polystyrene) 1, a pendulum 3 and a bracket 4. The principle of operation of the device is based on the movement of the pendulum 3 under the action of inertia forces that arise when the vehicle is braked. The amount of movement of the pendulum (inertial mass) is proportional to the deceleration during braking, i.e. than faster car slows down, the more the pendulum deviates from its initial (zero) position, and vice versa.
The amount of deviation of the pendulum from the zero position is indicated by arrow 6 on scales 2 and 15, graduated in deceleration values. On the lower scale, in addition to deceleration, determine the magnitude of the slope of the road in%.
The pendulum 3 can swing freely on the axis 10 in the supports of the bracket 9. There are pins on each side of the pendulum, one of which 8 (located on the side of the rear wall) serves to move the arrow 6, and the other 13 - to hold the pendulum in the zero position.
Rice. Pendulum type decelerometer
The arrow 6 is fixed on the axis of the handle 7, which serves to move the arrow and set it to zero on the scale. On the side of the cover 14, on the axis of the handle 11, a leash 12 is fixed, which, when the handle is turned to the right, presses the pendulum pin against the stop on the inside covers, which eliminates the oscillation of the pendulum in the non-working position. When the handle is turned 90° to the left (to the end), the pendulum can oscillate freely.
When monitoring the braking efficiency of a car, a decelerometer is installed on the glass of a door or windshield car cabin with the help of a hinged bracket 4 and rubber suction cups 5 so that the direction of swing of the pendulum coincides with the direction of movement of the car.
Then, having released the nut 16 of the bracket, the pendulum is aligned with the zero division of the scale by turning the body, and the arrow is set to zero by turning the knob 7.
Upon reaching a speed of 30 km / h, the pendulum is released with handle 11 and the car is sharply braked; when the car slows down, the pendulum 3 deviates from the vertical position (zero) and drags the arrow 6 along with it. After the car stops, the pendulum returns to its original position, and the arrow remains fixed at the place of the greatest deviation of the pendulum, indicating the amount of deceleration on the scale. Comparing the obtained deceleration values with the normative values, the vehicle braking efficiency is judged. The price of one division of the scale is 0.5 m/s2, the accuracy of readings is ±0.5 m/s2, the limits of the instrument readings are 0 - 8 m/s2.
The third method involves checking the brakes on stands with running drums (rollers) or with dynamometer platforms and tapes. The effectiveness of the brakes in this case is evaluated by the magnitude of the braking torque or force on each individual wheel, as well as by the synchronism of their action.
On roller stands, the wheels of the car rotate forcibly from its engine through the transmission or through rollers rotating from the electric motor of the stand due to the friction forces that arise between the tires and rollers.
Stands with rollers rotating from an electric motor have found the greatest application. Stands of this type have two (rarely four) dyno carriages with twin rollers, on which the car is mounted. The carriages are mounted on the floor at the inspection ditch or on a metal overpass. The scheme of the carriage of one of the stand designs is shown in the figure. Two corrugated cast iron (or coated with cast basalt) rollers 7, mounted on bearings in the frame, are interconnected by a roller chain 12 and are driven by an electric motor 1 through a worm gear 2 and two pairs of cylindrical gears 4, 3, 5 and 6, mounted in a housing 8.
During their rotation, the rollers cause the wheels of the car resting on them to rotate. When the car brakes, a braking force is generated between the wheel and the grooved roller, which slows down the rotation of the wheel. With an increase in resistance to rotation of the corrugated rollers, gear 5, transmitting rotation to gear 6, will itself simultaneously rotate relative to the geometric axis of this gear and roller 7, dragging along body 8, which will rotate in bearings 9. The whole system will work according to the principle planetary gear. If the body is kept from rotating by lever 10, then the force applied to lever 10 will be in a certain ratio with the amount of braking torque acting on roller 7 and the wheel tire and will be fixed by dynamometer 11 or recorded as a diagram of the dependence of braking force and pressure force on the pedal . To determine the beginning of the blocking of the car wheels during braking, an auxiliary roller is installed between the main rollers, pressed against the car wheel by a torsion bar or spring. The roller rotates simultaneously with the car wheel and stops when the wheels lock. At this moment it lights up control light or the carriage motors are switched off.
Rice. Scheme of a brake carriage with corrugated rollers
The force on the pedals is determined using a hydraulic sensor mounted on the brake pedal, or a pneumatic thrust is used for this purpose.
The forces recorded by the dynamometers during the rotation of the wheels, with the wheels unbraked, will show the force of the rolling resistance of the wheels. As the pressure on the pedal increases, the braking force increases proportionally until the moment the wheels start to slide along the brake roller, which corresponds to the maximum value of the braking force for a given grip weight per wheel.
Stands with movable platforms to control the brakes in the static state of the car have not received distribution.
In foreign practice, at service stations, stands with dynamometer platforms are used to control the brakes of a moving car, i.e. taking into account the manpower of the car. The stand consists of four corrugated platforms located at the floor level, and a measuring column located on the side of the platforms. During the test, the car is accelerated to a speed of 10-13 km / h and when it hits the sites, it is sharply braked. Under the influence of the inertia force acting on the car, and the friction force between the tires and the corrugated surface of the sites, they tend to move in the direction of the car.
The movement of each platform is transmitted by means of a system of rods, torque arms and hydraulic devices to an independent indicator of the measuring column.
According to the indications of the pointers, one can judge the uniformity of the distribution of the braking force on the individual wheels of the car and the magnitude of their braking force.
- We clean the tank from dirt.
- Visually check the fluid level in the brake reservoir.
- Check brake pad wear.
- If there is no wear on the pads, and the fluid level is below the MIN mark, then disconnect the wires from the brake fluid level emergency drop sensor, remove the reservoir cap
- Add brake fluid to reservoir up to max.
- We close the tank.
- Connect the wires to the sensor.
- To check the emergency brake fluid level sensor, press the rubber lining of the reservoir from above. If the sensor is working, the control lamp on the instrument brush will light up.
Checking the brake system also includes checking the vacuum brake booster:
- We install the car on the handbrake.
- With the engine off, press the brake pedal several times until the hissing in the brake booster stops.
- Keep the brake pedal depressed.
- We start the engine.
- If the brake pedal immediately moves down a little, the booster is working.
Otherwise, check the hoses going to vacuum booster for tightness.
The next check of the brake system is to check the parking brake.
Lever travel on a 23% grade should be a maximum of 4 clicks. If this is not the case, adjust it or replace the pads.
Check brake pedal free play.
The brake pedal free play is the stroke from top position before operation i.e. about 3-5 mm.
To do this, you need a ruler or tape measure.
Verification steps:
- We install the car on the handbrake.
- We install a ruler near the brake pedal and measure the distance from the floor to the brake pedal. Pressing the brake pedal with your hand, lower it until resistance increases.
We do this 3 times.
- We install the car on a viewing ditch.
- We remove the wheels
- When checking the left front pads, turn the steering wheel to the left and vice versa.
- We check the thickness of the pads, less than 1.5 mm.
Checking the wear of the rear brake pads is carried out in the following sequence:
- We install the car on the handbrake.
- Using a screwdriver, remove the rubber plug in the support shield of the brake mechanism.
- Light the hole with a flashlight. If the pad thickness is less than 1.5 mm, replace it.
- Install the plug back.
Checking the effectiveness of the brake system
In accordance with clause 1.1 of the "List of malfunctions and conditions under which operation is prohibited Vehicle» braking distances car with a single press on the brake pedal from a speed of 40 km / h should be no more than 12.2 m.
The test is carried out on a dry clean horizontal section of the road with a concrete or asphalt surface.
The brake system of the car must be tight.
The brake fluid level in the reservoir must be at the MAX mark.
1. Accelerate the car up to 40 km/h.
2. Press the brake pedal to the stop, blocking the wheels, and hold it in this position until the car comes to a complete stop.
3. Measure the stopping distance of the car with a tape measure. If the length is more than 12.2 m, the operation of the brake system is not effective enough.
4. To eliminate the influence of side effects, the braking distance is checked by accelerating the car in two opposite directions in the same section
Checking the brake fluid level
The brake fluid level is checked periodically: daily during vehicle operation, at each maintenance, after carrying out operations to bleed the hydraulic drive of the brake system and replace the brake fluid, in case of fire on the instrument panel control lamp, signaling insufficient level brake fluid in the master cylinder reservoir.
2. With a rag we remove dirt from the reservoir of the main brake cylinder.
3. Visually check the level of brake fluid in the reservoir. It must be between MIN marks and MAX on the tank body.
4. We check the degree of wear of the brake pads of the front and rear brake mechanisms.
5. If the wear of the brake pads is within normal limits, and the fluid level in the reservoir is below the MIN mark, then disconnect the tip of the wiring harness from the brake fluid level emergency drop sensor, unscrew and remove the reservoir cap.
6. Add new brake fluid to the reservoir to the MAX mark (when installing the reservoir cap, the sensor float will sink into the liquid and its level will rise).
7. Tightly close the tank lid.
8. Connect the wiring harness block to the sensor connector.
9. We check the operation of the brake fluid emergency level sensor: with the ignition on, we press on top of the rubber lining of the reservoir lid. If the sensor is OK, the control lamp on the instrument panel will light up.
Checking brake pad wear
Checking the degree of wear of the brake pads of the front brake mechanisms is carried out in the following sequence.
1. We install the car on a viewing ditch.
2. Remove the front wheels.
3. When checking the pads of the left brake mechanism, we completely turn steering wheel to the left, and when checking the pads of the right brake mechanism - to the right.
4. Through the inspection hole in the caliper movable bracket, we visually determine the thickness of the brake pad linings.
If the lining thickness of at least one pad is less than 1.5 mm, we change all brake pads of the brake mechanisms of the right and left wheels.
5. At the same time, we check the mobility of the pistons of the brake cylinders.
When the piston is soured, we replace the cylinder.
A flashlight is required to perform a wear check on the rear brake pads.
1. We prepare the car for work.
2. Using a slotted screwdriver, remove the rubber plug of the inspection hole in the rear wheel brake support shield.
