a hydraulic booster comprising a booster piston , an annular piston bearing against the booster piston and a central piston which is movable within limits relative to the annular piston by the brake pedal to control flow of a pressurized fluid . the central piston and the annular piston are coupled together to move as a unit after the force of a spring is overcome . the central piston is subject to the pressure acting on the booster piston . in order to reduce the force reaction on the brake pedal , the central piston is operated by the brake pedal through a pin subjected to the pressurized fluid , the pin extending centrally through the annular piston in a sealed relationship thereto .