Abstract: In a hydraulic brake booster, wherein a body, to which is coupled a master cylinder, is provided with a projecting portion extending outwardly towards the master cylinder; a power piston adapted to operate the piston of the master cylinder extends through the projecting portion to define a working chamber positioned on the side remote from the master cylinder; a pressure chamber, to which is introduced a pressure fluid from a pressure source, is provided in the middle portion of the body; and a first valve which is normally closed for controlling the communication of the pressure chamber with the working chamber and a second valve which is normally open for controlling the communication of the working chamber with a reservoir are operated by means of a valve stem member and an input shaft adapted to operate the valve stem member; the improvement in which a stop slidingly provided within the pressure chamber and adapted to receive pressure from the pressure chamber in the direction opposite to the moving direc