A pneumatic brake booster with a booster housing is subdivided into at least one working chamber and at least one vacuum chamber by means of at least one axially movable wall capable of being acted upon with a pneumatic differential pressure. An actuable input member includes a valve piston, with an output member for acting upon a brake master cylinder with an output force. A control valve is arranged in a control housing and can be actuated by the valve piston for controlling the differential pressure. An elastic reaction element is arranged in a control housing recess and against which the output member bears. Means for setting a distance z between the valve piston and the reaction element are provided on the output member.