The causes of a malfunction of the main brake cylinder are wear or loss of elasticity of the cuffs, wear of the working surfaces of the cylinder and pistons, swelling of the cuffs from mineral oils entering the system, clogging of compensation holes

If the fluid level in the reservoir of the main brake cylinder has decreased, and when examining the connections of pipelines and assemblies, including brake mechanisms, no brake fluid leak is detected, then in this case brake fluid may leak through the outer cuff 8 (see Fig. 1) into cavity A1 of the vacuum booster.

Brake Master Cylinder

Fig. 1. Brake Master Cylinder

If the brake pedal, when a force of about 200-300 N (20-30 kgf) is applied to it, first moves approximately halfway, and then gradually moves to the cabin floor under the same force, then in this case the main cuffs 14 are damaged or separating 16.

Defects in the separating cuffs can be detected; for this, liquid should be drained from the tank below the level of the separating partition by 10-15 mm in each section.

If, after pressing the brake pedal 3-5 times, the fluid level in the sections changes, this indicates fluid overflow from one section to another, which is possible only when the dividing cuffs are worn.

In the case of swelling of the cuffs, as a rule, the system is not released due to the overlapping of 14 compensation holes by the main cuffs. To determine this malfunction, it is enough to disconnect the tubes from the master cylinder.

If, after the liquid flows out of the working cavities, the leakage stops and the level in the tank does not decrease, then the compensation holes are blocked by cuffs or clogged. In these cases, the master cylinder should be removed from the vehicle and disassembled.

Removal and disassembly of the brake master cylinder must be done in the following order:

Gazelle master cylinder

To prevent dirt from entering the brake master cylinder, carefully clean the piping connections to the brake master cylinder.

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. Seal pipe openings.

Master cylinder and vacuum

Remove the bolt 1 securing the bracket 2 of the booster to the mudguard.

Unscrew the two nuts 3 securing the brake master cylinder 4 to the vacuum booster, remove the master cylinder with the bracket. At the same time, make sure that the rubber seal between the cylinder and the vacuum booster does not fall out.

Invert the master cylinder 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.

- disconnect the reservoir from the main brake cylinder and remove the connecting rubber bushings 3 with tubes 2 from the cylinder body;

- unscrew plug 20, remove spring 19 with thrust washer 18. Press piston 10, after which piston 17 with cuffs 14 and 16 can be removed by hand;

- remove the retaining ring with 11 pliers 7814-5593 or special pliers;

Main cylinder primary piston

Fig. 4. Master cylinder primary piston

- remove the piston 10 assembly by the shank by hand.

Remove the guide sleeve 9, the outer collar 8 and the thrust ring 7 from the piston. It is not recommended to unscrew the screw 3 (fig. 4) of the holder without the need.

After disassembly, it is necessary to carefully inspect the parts of the master brake cylinder and make sure that the cylinder mirror and piston working surfaces are completely clean and free of rust, marks and other defects.

In case of defects that cause a significant change in the inner diameter of the cylinder, or if it is worn on one side, replace the body with a new one.

It is recommended to replace the rubber cuffs with new ones every time the master brake cylinder is disassembled.

Before assembling the brake master cylinder wash all parts in alcohol or clean brake fluid. Do not allow mineral oils, gasoline, kerosene or diesel fuel to come into contact with the parts, as this may damage the rubber seals.

To assemble the brake master cylinder you need:

Installing the cuffs of the main brake cylinder

Fig. 5. Installation of cuffs of the main brake cylinder

Installing the master cylinder seals

- install the cuffs on the pistons, as shown in fig. 5;

- lubricate the cylinder mirror with brake fluid;

- assemble the primary piston 6 (see Fig. 6), check the size (19±0.275) mm (not adjustable);

- lubricate the cuff 14 (see Fig. 1) with fresh brake fluid and insert the piston 10 into the main cylinder body;

- insert thrust ring 7, outer cuff 8, guide sleeve 9 and retaining ring 11 using tongs or pliers;

- install separating cuffs 16, piston washer 13, main cuff 14, thrust washer 18 and spring 19 on piston 17;

- lubricate the cuffs with fresh brake fluid and insert the piston into the cylinder body;

- overcoming the resistance of the spring 19, screw the plug 20 with the gasket;

- install connecting bushings 3 with tubes 2 in the cylinder body, having previously lubricated them with fresh brake fluid;

- install the tank 4 on the connecting sleeves 3;

- install the main brake cylinder on the vacuum booster, having previously checked the condition of the sealing ring 12, and if necessary, replace it;

- pour brake fluid into the reservoir and bleed the system. Before installing new parts in the master brake cylinder, the preservation grease must be carefully removed from the latter to prevent it from entering the brake system.