Clutch servos

A clutch servo for a vehicle cable-operated clutch system has a housing, an input member to which an input force is applied by the cable which has an inner cable and an outer sheath, an output member connected to a clutch, and a movable wall for augmenting the output force which acts through the output member. The movable wall is operated by a fluid pressure chamber, pressurization of which is controlled by a valve in response to the input force. In one construction the inner cable is movable relative to the movable wall and a locking device is incorporated to lock the inner cable and the movable wall against relative movement when the movable wall is not in its retracted position. This ensures that the servo is not affected by adjustment of the cable to compensate for clutch lining wear when this adjustment is performed between the servo and the pedal. In another construction the servo includes an adjuster mechanism in the form of a locking mechanism between the housing and an end of the sheath, which is inoperative when the inner cable is retracted to allow relative movement between the end and the housing, and which operates when the inner cable moves from its retracted position to lock the sheath and the housing against movement. A valve actuating mechanism in the form of a lever is also incorporated to transmit the input force from the cable to the valve operation of which is controlled by the input force and an opposing force in at least one spring.