Brake master cylinder

The main brake cylinder is the central structural element of the working brake system. It converts the force exerted on the brake pedal into hydraulic pressure in the brake system. The operation of the brake master cylinder is based on the property of brake fluid not to be compressed by external forces.

On modern cars, a two-section brake master cylinder is installed. Each section serves its own hydraulic circuit. For front-wheel-drive cars, one of the circuits combines, as a rule, the brakes of the right front and left rear wheels, the second – of the left front and right rear wheels. In rear-wheel-drive cars, the working brake system is built in a slightly different way. The first circuit serves the brakes of the front wheels, the second – the rear wheels.

The main brake cylinder is mounted on the cover of the vacuum brake booster. Above the cylinder, there is a two-section reservoir with a supply of brake fluid, which is connected to the sections of the main cylinder through compensation and bypass holes. The tank serves to replenish the fluid in the brake system in case of small losses (leakage, evaporation). The walls of the tank are transparent, they are made reference marks, which allows you to visually monitor the level of brake fluid. A brake fluid level sensor is also installed in the reservoir. When the brake fluid level falls below the value set on the instrument panel, the warning lamp lights up.

In the main brake cylinder body, two pistons are arranged one after the other (in tandem). The rod of the vacuum brake booster rests against the first piston, the second piston is installed freely. The piston seal in the cylinder body is made using rubber cuffs. The return and retention of the pistons in the initial position is ensured by two return springs.

The principle of operation of the brake master cylinder

When braking, the rod of the vacuum brake booster pushes the first piston. When moving along the cylinder, the piston closes the compensation hole. The pressure in the primary circuit begins to rise. Under the influence of this pressure, the second circuit moves, the pressure in the second circuit also begins to increase. The voids formed during the movement of the pistons are filled through the bypass hole with brake fluid. The movement of each of the pistons occurs as long as the return spring allows. In this case, the maximum pressure is created in the circuits, which ensures the operation of the brake mechanisms.

At the end of braking, the pistons under the action of return springs return to their original position. When the piston passes through the compensation hole, the pressure in the circuit is equalized to atmospheric pressure. Even if the brake pedal is released sharply, vacuum in the working circuits is not created. This is prevented by brake fluid that fills the cavities behind the pistons. When the piston moves, this fluid smoothly returns (bypasses) into the tank through the bypass hole.

If brake fluid leaks in one circuit, the other circuit will continue to operate. For example, in the event of a leak in the first circuit, the first piston will move freely along the cylinder until it contacts the second piston. The second piston begins to move, providing the braking mechanisms in the second circuit.

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