Residual Check Valve
Many master cylinders designed for drum brakes have a residual check valve to keep a slight pressure on the system after the brake pedal has been released. It keeps around 5 - 12 psi. of pressure on the wheel cylinder's cup seals.
Since the valve keeps constant pressure on the system, any small leaks result in fluid leaking out of the wheel cylinder instead of air entering. Residual check valves keep constant pressure on the wheel cylinder's cup seals.
They're inside the master cylinder port leading to the rear drum brakes. They consist of a check valve and spring that allow brake fluid to enter the master cylinder only until a designated pressure is reached. Once this occurs, spring pressure closes the valve, maintaining low pressure on the drum brakes. Many vehicles today use cup expanders in the wheel cylinders that keep an airtight seal. They prevent air contamination, eliminating the need for residual check valves.