Brake booster with alternate activation by rotary electric motor

A vacuum booster for a vehicle brake system provides a working pressure valve seat on a valve member axially movable with respect to an input member. A rotary electric motor within the booster has an output shaft coupled through rotary to linear motion conversion apparatus to move the valve member with its working pressure valve seat relative to the input member. The combination of elements is designed as a drop-in replacement for a standard air valve member. The motion conversion apparatus provides much greater mechanical force than a linear solenoid; and this allows a more straightforward design of the valve arrangement, particularly the use of a high friction seal such as a simple "O"-ring in a groove. In further contrast to a linear solenoid, it further provides positive control of the working pressure valve seat position and an extended range of travel with constant force throughout the range. A magnet may be mounted on the nut adjacent a stationary magnetic sensor mounted on a circuit board associated with the motor to provide a signal of the position of the nut for the accurate positioning of the working fluid valve seat. A retaining clip normally used to prevent axial withdrawal of the input member from the power piston may be advantageously used to circumferentially engage an extension of a portion of the input member to prevent its rotation.