Abstract: There is disclosed a two-way valve which includes a single valve seat for controlling the flow of fluid in opposed directions through the valve responsive to differential fluid pressures across the valve. The valve includes first and second ports and a valve chamber. Across the valve chamber there is disposed a resilient diaphragm having an opening defining the valve seat. A ball valve element is supported in aligned relation to the valve seat and biasing means urge the ball element and valve seat into sealing engagement. When the pressure at the first port is greater than the pressure at the second port by a predetermined amount, the ball element moves away from the valve seat permitting fluid flow from the first port to the second port. When the pressure at the second port is greater than the pressure at the first port by a predetermined amount, the valve seat moves away from the ball element permitting fluid flow from the second port to the first port.