Each piston has a return spring in front of it. This holds the primary piston cup slightly behind the compensating or replenishing port (Figure ,46-10) for the reservoir allowing gravity to keep the cylinder filled with fluid. In addition to keeping the replenishing ports uncovered,
the return springs also help to return the brake pedal when the force has been removed from it. As the brakes are applied (Figure 46-11), the stiffer primary piston spring pushes the secondary piston and spring slightly. Then the cup at the front end of the secondary piston passes and closes off the primary replenishing port on the secondary side of the master cylinder.