The heart of the hydraulic brake system is the master cylinder (Figure 46-8). It converts the mechanical force of the driver's foot on the brake pedal to hydraulic pressure. The master cylinder has a bore that contains an assembly of two pistons and a return spring. One piston. pressurizes one-half of the brake system and one takes: care of the other half. Although master cylinders differ in , the size of the pistons, reservoir design, and integrated hydraulic components, the operation of all master cylinders is basically the same.