Vacuum servo unit for use in traction control

A servo unit for use in a vehicle braking system, the servo unit comprising a housing within which a flexible diaphragm separates a vacuum pressure chamber from an atmospheric pressure chamber, an actuator rod assembly extending through the housing and including a pedal for normal operation of brakes and a valve assembly having a valve control piston which has a laterally extending key, said valve control piston together with a power piston connected to the flexible diaphragm, being engageable with a valve closure member, said valve control piston together with the valve closure member controlling a first communication of the atmospheric pressure chamber with the atmosphere, and said power piston together with the valve closure member controlling a second communication between said atmospheric pressure chamber and said vacuum pressure chamber, said laterally extending key being engageable with a fixed stop in the housing to limit movement of the valve control piston and cause a balance to be set up on the valve assembly between the pressure in the vacuum pressure chamber and the pressure in that atmospheric pressure chamber when the actuator rod assembly is moved in a direction which would release the braking system, a solenoid valve being controllable in one condition to connect a flexible passage member extending around said actuator rod assembly across the vacuum pressure chamber between the housing and the power piston, to said vacuum pressure chamber for normal braking upon movement of the actuator rod assembly, or in response to a signal, being controllable to connect both said flexible passage member and a bypass leading from the solenoid valve to the atmospheric pressure chamber, to the atmosphere independently of said pedal for automatic brake application, said bypass incorporating a one-way valve to positively retain the pressure in the atmospheric pressure chamber under normal braking.