Abstract: A piston pump having a piston axially movable against the force of a spring within an operating chamber connected via check valves to an operating cylinder and a hydraulic medium supply. A section of the piston that extends into the operating chamber has a reduced diameter extension which extends from a shoulder of the piston that delimits the operating chamber. The extension includes a thickened free end having a sealing surface facing the shoulder. A valve disk is located and guided on the extension in a gap between the shoulder and sealing surface and is capable of axially reciprocating movements thereon. The valve disk is provided with openings which provide a passageway for hydraulic medium from the operating chamber to a second check valve. The openings are blocked when the valve disk abuts the sealing surface.