Scheme of brake operation of VAZ-2104 and VAZ-2105 cars (VAZ-2105)

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The entire cycle of brake operation consists of four positions of the brake pedal:

• I - pedal not pressed (the system is disengaged);
• II - pedal pressed (braking);
• III - pressing the pedal is suspended (constant torque braking);
• IV - the pedal is released (disinhibition).

I. When the system is released and the brake pedal is pulled back to the stop against the brake light switch by the action of the spring 32, then the pusher 31 with the piston 26 of the vacuum booster is pulled back together with the pedal. The valve body 20 and the rod 17 are pressed by the spring 19 to the extreme rear position. In this position, a gap is formed between the valve head 27 and the valve seat, since the piston 26 presses the valve away from the seat. The vacuum cavity A communicates with the atmospheric cavity D through channel B, the gap between the seat and the valve, and then through channel C. Therefore, when the engine is running, the vacuum from the engine intake pipe is transmitted through valve 18 to cavity A and through the channels and gaps to cavity O.

Pistons 11 and 15 of the master cylinder are pressed to the rear extreme position under the action of the return springs until they stop against the locking screws 8. In this position, the spacer bushings 13, resting against the screws 8, press the sealing rings 9 away from the end of the piston groove and through the resulting gaps the working cavities of the cylinder communicate with the hydraulic cylinder reservoir and high-pressure pipelines. Thus, there is no pressure in the brake drives. Therefore, pistons 4, under the action of the elastic deformation of the sealing rings 3, are retracted into the cylinders and do not exert pressure on the brake pads of the front brakes, which will be in light contact with the surface of the brake disc.


When the vehicle is moving without braking, that is, when there is no pressure in the hydraulic drive, piston 36, under the action of spring 40 and torsion lever 42, is raised up to the stop against plug 35. Therefore, the cavities of the housing, located above and below the piston head, communicate freely.

This opens a free passage of fluid to the wheel cylinders of the rear brakes. But since there is no pressure in the entire brake drive, the brake shoes 43 are pressed away from the drums, and the pistons 44 are pushed inside the wheel cylinder until the crackers stop against the flanges of the thrust rings 46.

II. When braking, when the driver presses the brake pedal, the pusher 31 moves the piston 26. Following the piston, the valve 27 moves under the action of the spring 28 until it stops against the valve body seat. When the seat is closed, cavities A and B are disconnected. With further movement of the piston 26, a gap is formed between it and the flange of the valve 27, through which cavity O communicates with the atmosphere. Outside air enters cavity B through the air filter 30, through the gap between the pusher and the valve and then through channel C. Atmospheric air creates pressure on diaphragm 21. Due to the difference in pressure in cavities A and D, as well as the force of pressing the brake pedal, the valve body moves together with the rod 17, which in turn acts on the piston 15 of the master cylinder. The force acting on the valve body depends on the degree of vacuum in the engine intake pipe and on the force applied to the brake pedal.

When piston 15 moves, spacer sleeve 13 moves away from lock screw 8 and sealing ring 9 is pressed by spring 12 to the end face of the piston groove. Thus, the compensation gap is closed and the cavities of the cylinder and tank are separated. Therefore, with further movement of piston 15, fluid pressure is created in the working cavity of the front brake drive, which is transmitted through pipelines and hoses to the wheel cylinders of the front brakes. It also acts on floating piston 11, which, moving, creates pressure in the rear brake drive. Under the increasing pressure of the fluid in the working cavities, the front piston sealing rings expand and begin to fit more tightly to the surface of the cylinder and to the end face of the grooves, improving the sealing of the pistons in the cylinder.

Under the pressure of the liquid, pistons 4 and 44 of the wheel cylinders of the front and rear brakes are extended, pressing the shoes to the brake disc 1 and to the drum. The created braking moments slow down the rotation of the front and rear wheels. At the same time, the load is redistributed along the axles of the car: the load on the front axle increases, on the rear axle it decreases. This leads to the lifting of the rear of the body, i.e. the distance between the rear axle beam and the body increases. At the same time, the short arm of the lever 42 is lowered, and the piston 36 of the pressure regulator under the pressure of the liquid begins to descend, compressing the spring 40.

At the moment of full braking, the maximum load transfer from the rear axle to the front axle and the greatest lift of the body occur. The adhesion of the wheels to the road deteriorates, the pressure of the torsion lever 42 on the piston 36 decreases. Due to the larger area of the piston head end, the force from the pressure P ₂ of the liquid lowers the piston down until the head contacts the seal 38. Further flow of liquid to the wheel cylinders of the rear brakes stops, i.e. the braking torque on the rear wheels does not increase, despite strong pressure on the brake pedal and a further increase in pressure. Therefore, the rear wheels do not lock and the car does not skid.

III. If the driver stops pressing the pedal while braking, but leaves it pressed in some position without removing his foot, then the vacuum booster body will move forward under atmospheric air pressure by the amount of the gap between plate 25 and the piston groove, i.e. it will move away from the valve. The released valve, moving, will reach the stationary piston and block the air flow into cavity D, and the excess air in cavity D will pass into vacuum cavity A through the resulting gap between the seat and valve 27 and channel B. The pressure in both cavities will equalize and the servo action of the booster will stop. At some point, a constant pressure is established in the brake drive circuits, and a constant braking torque is established on the wheels.

IV. When the brake pedal is released, it returns to its original position under the action of the return spring 32, dragging the pusher 31 and piston 26 along with it. The rear end of the piston is pressed against the valve head 27, which leads to the cessation of the flow of atmospheric air into cavity B. Then the valve head moves away from the seat and communication between cavities A and B occurs, i.e. the pressure in both cavities is equalized; under the action of spring 19, the valve body with the stem returns to its original position, stopping the pressure on piston 15 of the main cylinder.

Pistons 11 and 15 are pressed to the extreme position by the force of the return springs and rest against the locking screws 8. The spacer bushings 13 move the sealing rings 9 away from the end of the grooves and through the resulting gap the working cavities of the master cylinder communicate with the cavities of the hydraulic cylinder reservoir. Pistons 4 of the front brake are moved away from the shoes due to the elasticity of the sealing rings 3, and pistons 44 of the rear brake are moved away by the reduction of the tension springs until a gap of 1.4-1.6 mm is selected between the crackers and the flange of the thrust ring 46.

If the rear brake drive circuit fails due to its leakage, piston 11 moves under fluid pressure until it hits the plug of the master cylinder, after which the pressure in the front brake drive circuit begins to increase. Due to the free movement of piston 11, the free travel of the brake pedal increases and only the front brake drive operates.

When the front brake drive circuit fails, piston 15 moves forward until it stops against piston 11, after which the rear brake drive circuit begins to operate. The free travel of the brake pedal also increases.

It should be remembered that when increasing the free travel of the brake pedal, it is not recommended to press the pedal repeatedly, as this will not speed up braking, but on the contrary, will lengthen the brake response time. You should continue to press the pedal all the way and, if necessary, apply the parking brake.

If any brake drive circuit is damaged, the fluid level control lamp lights up, signaling a drop in the fluid level in the reservoir.

The parking brake system acts on the rear wheel brakes via a mechanical drive. When lever 47 (see fig. 27) upwards, after selecting the free travel of the lever, equal to 3-4 clicks, the drive cable is tensioned and the force is transmitted to the levers 21 (see fig. 26) manual drive of the shoes. When turning the lever 22 on the finger 23, the force through the expansion bar 37 is first transmitted to the front brake shoe until it is completely pressed against the drum. After which the lever 22 moves relative to the point of contact with the expansion bar, and its upper arm presses the other shoe against the drum. At the same time, the control lamp on the instrument cluster lights up with a red flashing light, since the lever stop moves away from the lamp switch rod, and the circuit is closed.