If one of the circuits of the service brake system fails, the second circuit is used, ensuring that the vehicle stops with sufficient efficiency.
The hydraulic drive includes a vacuum booster and a dual-circuit rear brake pressure regulator.
The parking brake system has a cable drive to the rear wheel brake mechanisms.
9.1. Front wheel brake mechanism: 1 – brake disc; 2 – brake pads; 3 – brake mechanism support; 4 – shoe guide; 5 – caliper guide pin; 6 – air release valve; 7 – brake hose
The front wheel brake mechanism is disc, with automatic adjustment of the gap between the pads 2 (figure 9.1) and a disk 1, with a floating bracket. The movable bracket is formed by a support 3 with a single-piston working cylinder. The guide 4 of the pads is bolted to the steering knuckle. The movable bracket is bolted to the guide pins 5 installed in the holes of the guide of the pads. The guide pins are lubricated with consistent grease and protected by rubber boots. A piston with a sealing ring is installed in the cavity of the wheel cylinder. Due to the elasticity of this ring, an optimal gap is maintained between the pads and the disk, the surface of which is protected by a brake shield. When braking, the piston, under the influence of fluid pressure, presses the inner pad to the disk, as a result of the reaction force, the caliper moves on the pins and the outer pad is also pressed to the disk, while the pressing force of the pads is the same. When releasing the brakes, the piston is moved away from the pad due to the elasticity of the sealing ring, and a small gap is formed between the pads and the disc.
Master brake cylinder 1 (figure 9.2) the "tandem" type of hydraulic brake drive consists of two separate chambers connected to independent hydraulic circuits. The first chamber is connected to the right front and left rear brake mechanisms, the second - to the left front and right rear.
9.2. Master brake cylinder with reservoir: 1 – main brake cylinder; 2 – master brake cylinder reservoir; 3 – tank cap; 4 – connecting bushings
A tank 2 is installed on the master cylinder via rubber connecting bushings 4, the internal cavity of which is divided by a partition into two sections. Each section feeds one of the chambers of the master cylinder.
When you press the brake pedal, the pistons of the master cylinder begin to move, the working edges of the cuffs cover the compensation holes, the chambers and the reservoir are separated and the displacement of the brake fluid begins.
A brake fluid level sensor is installed in cap 3 of the tank. When the fluid level drops below the permissible level, a brake system malfunction indicator lamp lights up in the instrument cluster.
Vacuum booster (figure 9.3), installed between the pedal mechanism and the main brake cylinder, during braking, due to the vacuum in the engine intake pipe through the rod and piston of the first chamber of the main cylinder, it creates additional force proportional to the force from the pedal.
A check valve is installed in the hose connecting the vacuum booster to the intake pipe. It maintains the vacuum in the booster when it drops in the intake pipe and prevents the fuel-air mixture from entering the vacuum booster.
9.3. Vacuum amplifier
The pressure regulator changes the pressure in the hydraulic drive of the rear wheel brake mechanisms depending on the load on the rear axle of the car. It is included in both circuits of the brake system, through it the brake fluid goes to both rear brake mechanisms. The regulator is fixed with bolts to the car body. Its rod through a spring-loaded loading rod, lever 3 (figure 9.4) and the earring 5 is connected to the rear suspension beam. Depending on the distance between the beam and the body, which is determined by the vehicle load, the regulator rod moves, which, in turn, with the help of a valve system, changes the cross-sections of the circuit passages inside the regulator, thereby limiting the pressure in the rear brake circuits. The degree of regulator limitation, and therefore the pressure in the circuits, is adjusted by changing the length of the regulator rod using nut 4.
9.4. Rear brake hydraulic pressure regulator: 1 – pressure regulator body; 2 – regulator rod cover; 3 – lever; 4 – adjusting nut; 5 – earring
The rear wheel brake mechanism is a drum brake with automatic adjustment of the gap between the shoes and the drum. Brake shoes 1 and 11 (figure 9.5) are driven by one hydraulic working cylinder 8 with two pistons. The optimum clearance between the drum and the shoes is maintained by a mechanical regulator 6 mounted on the spacer bar 9.
9.5. Rear wheel brake mechanism: 1 – front brake shoe; 2 – brake mechanism shield; 3 – spring of the clearance adjuster lever; 4 – support posts; 5 – clearance adjuster lever; 6 – clearance adjuster; 7 – upper tension spring; 8 – working cylinder; 9 – spacer bar; 10 – parking brake release lever; 11 – rear brake shoe; 12 – Parking brake drive cable; 13 – lower tension spring
The mechanically operated parking brake consists of a lever mounted on the base of the body between the front seats, a front cable with an adjusting device and equalizer, to which two rear cables are connected, and release levers installed in the brake mechanisms of the rear wheels.
The parking brake does not require special maintenance. During routine maintenance, check the wear of the sector teeth and pawl. Replace excessively worn parts.
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If any breaks in the cable sheaths or wires are detected, they must be replaced with new ones.
The anti-lock braking system (ABS) consists of an electronic control unit (ECU), wheel speed sensors, a hydraulic unit 1 (figure 9.6) with hydraulic solenoid valves, electrically driven return pump and indicator light 4 in the instrument cluster.
ABS is used to regulate the pressure in the brake mechanisms of all wheels when braking in difficult road conditions, preventing wheel locking. The system provides the following advantages:
- avoiding obstacles with a higher degree of safety, including during emergency braking;
- reducing the braking distance during emergency braking while maintaining the vehicle's directional stability and controllability, including when turning.
In case of system failure, a function is provided to maintain operation during system failures.
The electronic control unit (ECU) receives information about the vehicle's speed, direction of travel and road conditions from the wheel speed sensors.
Based on this information, the control unit determines the optimal braking mode for the wheels, changing the flow sections of the circuits using electromagnetic valves in the hydraulic unit, anticipating the moment of locking the wheel that slows down rotation, thereby preventing it from locking.
If the system detects that any wheel is locked, it commands the appropriate valve to isolate the fluid supply to the wheel cylinder of that wheel from the master cylinder.
If the wheel speed continues to decrease relative to the other wheels, the ABS returns brake fluid back to the master cylinder, reducing braking. If all four wheels slow equally, the return pump will shut off and all solenoids will reopen, allowing the master cylinder to operate the wheel cylinders normally. This cycle can repeat up to ten times per second.
The activation of the electromagnetic valves and the return pump creates pulsations in the hydraulic drive of the brake system, which are transmitted to the brake pedal, thereby signaling to the driver that the ABS is working.
The solenoid valves in the front brake circuits act on their working cylinders independently, each separately, while the solenoid valve in the rear brake circuits acts on both working cylinders of the mechanisms simultaneously. Since the braking system has a diagonal division, a separate mechanical plunger valve in the hydraulic block divides the hydraulic output of the rear solenoid valve into two separate circuits. In order to prevent the system from being affected by false signals, an integrated safety circuit monitors all signals sent to the ECU. If a false signal is received or the voltage in the on-board electrical system is insufficient, the system is automatically switched off and the ABS deactivation warning lamp in the instrument cluster lights up. In this case, the normal mode of operation of the braking system is maintained, but extreme caution must be exercised when driving on slippery roads, since the distribution function of the system is impaired (function of equalizing pressure in the brake mechanisms of the front and rear wheels) and there is a risk of the car skidding when braking.
If a fault occurs in the ABS system, contact a service station, as special equipment is required for its diagnosis and repair.
The hydraulic brake system is connected into a single unit by metal tubes and hoses. The system is filled with a special brake fluid of class not lower than DOT-4, which must be replaced periodically. The procedure for replacing the brake fluid is described in Section. "Maintenance" (cm. "Replacing brake fluid").
Checking the brake system is described in the section "Maintenance", see. "First technical maintenance (TO-1)".
9.6. Anti-lock braking system diagram: 1 – hydraulic block; 2 – main brake cylinder; 3 – brake booster; 4 – ABS shutdown indicator light in the instrument cluster
Note. The working stroke of the brake pedal with the engine running should be approximately 60–65 mm. Too small a working stroke indicates incorrect initial installation of the brake pedal, incorrect adjustment of the brake booster or jamming of the working cylinder, causes increased fuel consumption and accelerated wear of the brake pads. Too large a working stroke is a sign of excessive clearances in the pedal mechanism or a leak in the hydraulic drive of the brake system. If the working stroke decreases with repeated pressing of the pedal, i.e. it becomes "harder", there is air in the system. If the full pedal stroke begins to increase, the system is leaky.
If the brake pedal always starts to vibrate when braking, the brake discs are most likely warped. Unfortunately, in this situation, they only need to be replaced, and both at once. Periodically appearing and disappearing vibration of the pedal during sharp braking accompanies the operation of the anti-lock braking system and is not a sign of a malfunction.
If the car starts to pull to one side when braking, check the wheel cylinders: they may need to be replaced.
If a knocking noise appears in the front suspension that disappears when braking, check the tightness of the caliper mounting bolts.
After replacing the brake pads, before starting to move, be sure to press the brake pedal several times - the pistons in the working cylinders should fall into place.