The main brake cylinder with two pistons 10 and 17 in series and a two-section reservoir 4 with a sensor 6 for an emergency drop in the brake fluid level is attached to the cover of the vacuum booster.

Brake Master Cylinder

The brake master cylinder pressurizes two independent hydraulic circuits.

The volume of brake fluid between pistons 10 and 17 is used to actuate the rear brakes, and the volume of fluid between piston 17 and the end of plug 20 is used to actuate the front brakes.

When braking, moving forward, the primary piston 10 and its cuff 15 block the compensation hole B connecting the primary cavity of the main cylinder with the reservoir.

The spring installed between the pistons 10 and 17 is stronger than the spring 19 located between the piston 17 and the plug 20, therefore, simultaneously with the primary piston 10, the secondary piston 17 also begins to move, blocking the compensation hole A connecting the secondary cavity of the cylinder with the tank.

Further movement of the pistons is accompanied by an increase in pressure in the cavities, and, consequently, both brake circuits are activated.

When the force is removed from the brake pedal, the pistons return to their original position under the action of the springs.

This opens the expansion holes A and В and the liquid in both cavities of the main cylinder communicates with the liquid in the tank, while the pressure in the circuits decreases to atmospheric pressure.

If the brake pedal is released abruptly, the master cylinder pistons quickly return to their original position and are ready for subsequent braking.

The quick return of the pistons is ensured by the fact that when they return to their original position, a vacuum is created in the cavities of the main cylinder, under the influence of which the liquid from the tank through the bypass holes C and the holes in the pistons, squeezing the washers 13 and the edges of the cuffs 14, enters the cavities of the main cylinder.

When the pistons reach their home position, excess fluid from each cavity through the expansion holes A and The will flow into the tank.

When one of the drive circuits fails, fluid leaks from the cavity of the main cylinder connected to the faulty circuit.

If the rear circuit is faulty, then the piston 10 reaches the spring holder 16 and through it acts on the secondary piston 17, which creates pressure in the secondary cavity of the master cylinder and the front circuit.

If the front brake circuit fails, piston 17 acts on the end of plug 20, and piston 10, compressing the spring, displaces fluid from the primary cavity of the master cylinder into the circuit going to the rear wheel brakes.

If one of the drive circuits fails, the brake pedal stroke increases, but the vehicle is sufficiently effectively braked.

With good circuits, the brake fluid level in the master cylinder reservoir should be between the MAX and MIN marks.

The gradual change in the fluid level from MAX to MIN is due to the wear of the brake linings.

A sharp drop in the level of brake fluid in the reservoir indicates a violation of the tightness of the brake system.

At the same time, the brake fluid level warning light is activated and the red lamp on the instrument cluster lights up. in this case, add liquid only after restoring the tightness of the system.

To check the serviceability of the sensor 6 of the emergency drop in the liquid level in the tank, with the ignition on, press the central part of the protective cap 5 from above. In this case, the lamp on the instrument cluster should light up.

Removing the brake master cylinder

1. To prevent dirt from entering the master brake cylinder, carefully clean the places where the pipelines are connected to the brake master cylinder.

Brake Master Cylinder Repair

2. Disconnect connector 2 with wires from the brake fluid level sensor installed in cover 1 of the master cylinder reservoir and remove the cover.

Unscrew the fastening nuts 3 and disconnect the pipelines from the master cylinder. Plug the openings of the pipelines.

Removing the brake master cylinder

3. Remove bolt 1 securing bracket 2 for booster to mudguard.

Unscrew two nuts 3 securing the master brake cylinder 4 to the vacuum booster, remove the master cylinder with the bracket.

In this case, make sure that the rubber seal between the cylinder and the vacuum booster does not fall out.

4. Turn the master cylinder over and drain the brake fluid from the reservoir into a container. Then press the piston of the main cylinder several times, thereby removing the liquid from it.


1. Clamp the master cylinder in a vise and remove the rubber seal if it remains on the cylinder.

Disassembling the brake master cylinder

2. Remove the retaining ring while holding the piston.

Removing the brake master cylinder

3. Remove the primary piston assembly 1 from the cylinder.

Remove the cylinder from the vise and, gently tapping the end against a clean piece of wood, remove the secondary piston 7 and spring 6.

Remove the guide sleeve 2, the outer collar 3 and the thrust ring 4 from the primary piston.

It is not recommended to unscrew the adjusting screw that secures the spring holder 8 without necessity.

Remove the cuff 5 from the secondary piston.

Removing the brake master cylinder

4. to replace the connecting sleeves 2 of the tank, it is necessary to remove the tank 1 and remove the connecting sleeves 2 with plastic tubes from the master cylinder

Inspection and troubleshooting of brake cylinder parts

Wash the master cylinder parts with clean brake fluid and dry.

brake master cylinder repair

The mirror of the main cylinder should not have scratches, shells and foci of corrosion. If these defects are present, the cylinder must be replaced.

There must be no mechanical damage on the threaded parts of the cylinder.

The cuffs of the master cylinder must be resilient and elastic. Torn, swollen and hardened cuffs, as well as those with marks on the working surfaces, must be replaced.

The end of the pusher of the vacuum booster and the mating surface of the primary piston should not have obvious signs of one-sided wear.

The surfaces of the pistons in contact with the cylinder mirror must also not show signs of one-sided wear. Otherwise, it is recommended to replace the pistons.

Springs must have elastic properties.

Broken springs are replaced.

The threads of the tube ends must not be mechanically damaged.

The fitting of the tips should not have cracks and traces of visible deformation.


Assembly of the master brake cylinder is carried out in the reverse order of disassembly.

Before assembling, it is necessary to lubricate the parts of the master cylinder with new brake fluid.

When installing the cuffs on the pistons, the working edges must be oriented as shown in the photo.

Install the cylinder in the reverse order of removal. After installation, it is necessary to bleed the brake system.