The pressure medium reservoir (1) may be used in the brake system for a motor vehicle. It has a cylindrical housing (2) which is closed at one end and has a stepped bore with the inner diameter of the portion near the closed end being slightly smaller than the inner diameter of the portion at the open end. A hollow piston (4) slides inside the housing and has first and second one-way seals (9,11) engaging the narrow and wide inner diameter portions of the bore and defining an annular pumping chamber. A first chamber (5) is formed in the annular space between the outer diameter of the piston and the end of the housing and a lid (3) with a bore (6) to admit fluid. A second chamber (7) is formed in the closed end of the housing a third chamber (10) is formed between an O-ring (8) and the second one-way seal. The piston has a cross-wall dividing off the second chamber from a fourth chamber.
