Normally used in braking systems on vehicles, a master cylinder is a tube or reservoir of fluid that supplies the pressure to a hydraulic system that eventually leads to actuation of the brakes. It is the first major component of the hydraulic system and is necessary for converting the movement from a brake pedal or lever into hydraulic pressure. In a car, the master cylinder is usually mounted in the engine compartment.
A hydraulic braking system works by using hydraulic force to actuate pistons. This is accomplished by pushing a piston through the master cylinder, which in turn pushes hydraulic fluid through hollow brake lines. The force is then transferred to a caliper, which has pistons inside it that press the brake pads against a rotating rotor. When the force is applied, the brake pads slow or stop the rotor. While this describes only one style of brake — called a disc brake — other types of hydraulic brakes work in a similar fashion.