Brake master Restrictor

Brake Master Cylinder 'Slow-return' Valve

A general view of the single-circuit master valve, which comes as part of the seal replacement kit, but is not shown in the Parts Catalogue. The three 'legs' go in the end of the piston return spring, and the flat face against the end of the cylinder over the outlet port.

The slot in the side of the valve, normally covered by a brass spring. When you depress the brake pedal fluid pushes this brass spring out of the way and flows freely into the brake system.

On the opposite side from the slot and the brass spring there is a small groove cut into the flat face of the valve. When the pedal is released the brass spring returns and closes the slot, so the only exit for fluid pressure is the small groove, which delays the return of the fluid. However all the fluid pressure is eventually dissipated back into the master, it does not maintain any residual pressure. It's this valve that allows you to 'pump up' the brakes with several quick presses when you have air in the system or the rear shoes or handbrake are very poorly adjusted.

The early dual master seems to have just one of these valves (although two are shown for other applications of the same master), referred to as 'body-trap valve' 27H8453.