Abstract: A hydraulic valve wherein the at least partially cylindrical internal surface of a cylinder or the at least partially cylindrical external surface of a piston in the cylinder is formed with a slot-shaped circumferentially extending outlet communicating with a plenum chamber. The other surface is formed with a circumferentially extending groove for a portion of a one-piece or composite sealing ring. The piston is movable in the cylinder between at least one first position in which the ring sealingly engages the slotted surface at one side of the outlet and at least one second position in which the ring sealingly engages the slotted surface at the other side of the outlet. That portion of the slotted surface which is adjacent the outlet is rounded, and that portion of the grooved surface which is adjacent the groove is also rounded.