Controller for valve mechanism of brake booster

A brake booster is provided in which a hydraulic pressure applied to a power piston is controlled by controlling a valve mechanism in response to a movement of an operating rod which is mechanically interlocked with a brake pedal. A valve plunger connected to the free end of the operating rod forms the valve mechanism, and is provided with a stop member, which limits a rear position of the valve plunger by abutment against the internal surface of a closed shell or a support mounted thereon when the brake booster is inoperative. As a consequence, the cross-sectional area of a flow path for the hydraulic fluid can be increased while maintaining an optimum depression of the brake pedal. Since such result is achieved by the stop member which is disposed within the closed shell, the arrangement can be constructed with a reduced weight and size as compared with a conventional arrangement in which the stop is disposed on the exterior of the shell.