Introduction master brake cylinder:
The master brake cylinder is mounted to the brake booster. On top is the brake fluid reservoir, which takes care of the supply and return of the brake fluid. When the brake pedal is pressed, fluid pressure is built up in the master brake cylinder via the vacuum action in the brake booster. From the master brake cylinder the pressure is transferred via the brake hoses and brake lines to the wheel brake cylinders.
These days we only use tandem master brake cylinders. These divide the brake system into 2 brake circuits:
- One circuit for the front wheels and one for the rear wheels.
- Diagonally separated brake circuits; in this case one circuit operates the left front wheel and the right rear wheel. The second circuit then operates the right front wheel and the left rear wheel.
- When using disc brakes with multiple pistons per caliper, each caliper can be operated by 2 separate circuits.
In the image below you can see a vehicle with a diagonally separated brake circuit. The red line indicates the brake circuit of the left front wheel and the right rear wheel. The blue line indicates that of the right front and left rear.
Operation:
In the image the tandem master brake cylinder is shown in its rest position. Braking is therefore not taking place now. The springs of the primary and secondary pistons push both the operating piston and the primary piston to the left, so that the space at the drillings 3 is opened up. The brake fluid reservoir is divided at the bottom into 2 separate sections. When part of the brake system leaks and one brake circuit runs empty, this has no effect on the other brake circuit. The braking action is maintained via the other circuit.
During braking:
The operating piston (1) moves to the right. Because fluid is not compressible, the fluid also moves the secondary piston. At the moment there is a leak in the primary brake circuit, mechanical contact is made between the primary and the secondary piston. When the primary cups have passed the compensation ports (3) (as in the image), pressure is built up. The brake fluid is forced to the wheel brake cylinders, causing the brakes to be applied. The further operating piston 1 is moved to the right, the higher the brake pressure becomes. Both pistons move to the right simultaneously, because the (left) spring behind the primary piston has a higher preload than the (right) spring next to the secondary piston. When braking is stopped, the operating piston moves back to the left and both springs push the primary and the secondary piston back to the basic position.
The image below shows an exploded view of the components in the master brake cylinder with the brake fluid reservoir.
Defects:
Malfunctions can be caused in the brake system by defects in the master brake cylinder. Below two situations are described that are characteristic of a defect in the master brake cylinder.
- Defect 1: When the brake pedal sinks ever deeper while a constant force is applied to the brake pedal, it seems as if there is a loss of pressure in the system. First you would think of a leak. When no leak can be found but the brake pedal still continues to sink, the problem may be in the master brake cylinder. The primary cup, which seals the space between the compensation ports (3) and the brake circuit, may be leaking. The brake fluid is forced under pressure into the brake circuit by the primary or secondary piston. Due to this leaking cup, the fluid slowly flows back through the compensation port into the brake fluid reservoir.
By replacing the brake cups the problem is solved, but usually the complete master brake cylinder is replaced for this.
- Defect 2: When the seal of the operating piston (all the way on the left-hand side in the image) leaks, it is possible that brake fluid from the master brake cylinder flows directly into the brake booster. There is a risk that the operation of the brake booster will be affected by this and that the primary brake circuit (1) will lose brake pressure due to a brake fluid level that is too low. In the event that all the brake fluid from the primary brake circuit has leaked away (with a diagonally separated system and the primary brake circuit only braking the left front wheel and the right rear wheel), then the secondary brake circuit will absorb all braking forces. Because the secondary brake circuit then only brakes the right front wheel and the left rear wheel, the braking effect will be considerably less. But the most important thing is that the car can still be brought to a standstill.