A brake booster in which a servo piston is slidably supported in a housing. The transmission of a brake force is effected via a piston rod protruding into the housing, and a work chamber is formed between the piston rod and the servo piston. After the opening of a valve, this work chamber can be filled with brake fluid from a reservoir or supply container via a pump or the like. A return line also leads out of the work chamber back to the supply container, via a further valve. To improve the closing or sealing behavior of the valves, the first valve is to be embodied for admitting the brake fluid from a valve cone, or its shoulder, the valve cone extending axially in a blind bore of the servo piston, and a valve seat formed out of a valve bushing. The second valve is likewise embodied as a seat valve, and an element connected with the valve cone cooperates with an element connected with the piston rod.