A valve having an annular inlet valve seat and an annular exhaust valve seat arranged concentrically and a flap valve diaphragm which is engageable at its outer peripheral portion with the inlet seat and at its central portion with the exhaust seat and which can flex to uncover either the inlet seat or the exhaust seat, a stem being secured to the flap valve diaphragm and to a second diaphragm and being arranged to balance the pressures acting on the flap valve diaphragm, of which the following is a specification.