Master Cylinder and Separate Braking Circuit Explained
Master Cylinder Operation
The brake master cylinder is responsible for providing adequate fluid pressure to the wheel cylinders, which in turn produce the surface application on the front wheels. The master cylinder receives brake fluid from a tank attached to it and can transfer it through conduits to the wheel cylinders. Inside the cylinder, a piston slides, fitted with a rubber cup housed in a groove. This cup performs the necessary sealing between the piston and the cylinder. A washer marks the top of the piston’s backward travel, supporting it in the rest position.
In front of the piston is the primary cup, positioned by a spring and a double-action piston valve. The piston is powered by the control rod, which is attached to the brake pedal at its other end. In the rest position, the chamber is full of fluid that enters through the compensation hole. This chamber is at barometric pressure due to its communication with the reservoir, which is subject to the same pressure. The spring remains retracted against the piston top and applied against the valve seat, preventing communication between the chamber and the wheel cylinder pipes behind the primary cup. Fluid enters the chamber, providing smooth gliding when the driver steps on the brake pedal.
When the brake pedal is pressed, it pushes the piston rod, which carries with it the primary cup. The cup is perfectly attuned to the cylinder walls, preventing fluid leakage backward. The fluid locked in the chamber is compressed during the piston’s displacement. Simultaneously, the spring applies increasing pressure against the valve seat. As long as the cup does not cover the compensation hole, a little fluid goes into the tank, providing compensation that prevents sharp braking control.
Once the compensation hole is plugged, the subsequent displacement of the piston raises the pressure in the chamber. At a certain point, the pressure is enough to open the valve, deforming its rubber edging and freeing the orifices through which fluid can leave the conduits. When the driver releases the brake pedal, there is an instantaneous pressure drop in the compression chamber. This causes the pump’s piston, under the pressure of its spring, to return more quickly than the wheel cylinder pistons.
Separate Braking Circuit
Brake systems usually consist of two independent hydraulic circuits that drive the front and rear brakes separately in most cases. This way, if there is a leak in the rear brakes, for example, the front brakes will continue to operate, and the vehicle will not be left entirely without brakes. Similarly, circuits can be arranged in an “X” configuration, where one circuit controls the front right and rear left brakes, and the other controls the remaining two. In either case, having two separate braking circuits requires a tandem pump.
A tandem pump comprises a cylinder in which two pistons are housed. The primary piston is driven directly by the brake pedal, while the secondary piston is driven by the action of the spring and the pressure in the chamber.