A fluid flow control valve has a diaphragm for sealing against a valve seat. The diaphragm includes two generally vertical annular lips extending upwardly and downwardly from a peripheral bead of the diaphragm. The bead is located at an interface between two mating portions of the valve with the lips received in annular channels located in the mating valve portions. The channels are configured relatively to the lips to bend the lips out of their normal, undistorted orientation to impose a biasing force on the lips tending to seal the lips in the channels.