A vacuum servo unit is provided with an actuator for controlling the distance between the engaging portion of a first input member with a reaction disc and an air pressure valve seat in response to a driving force. The vacuum servo unit further includes a rubber member disposed between the first input member and a movable core. The rubber member is deformable in response to the relative movement of the first input member and the movable member forming a part of the actuator. When the actuator is inactive, an axial space is formed between the reaction member and the first input member and between the first input member and a second input member.