The master cylinder is one of the main components of your vehicle's braking system. It converts the non-hydraulic pressure of your foot pressing down onto the brake pedal into hydraulic pressure, which in turn begins the braking process. Pistons inside the master cylinder are displaced by the pressure of your foot on the brake pedal, and in turn begin to apply pressure on the brake fluid in the lines that runs to the vehicle's wheels. In the brake calipers that grip the 4 rotors are pistons in which this pressure is exerted upon. This pressure causes the pistons to press outward, gripping the rotor surface and slowing the car down.