A gas flow valve having an internal valve chamber with an inlet and outlet passage, and a movable metallic diaphragm in the chamber, the diaphragm bonded to a valve stem projecting outside of the chamber. The diaphragm has a Teflon film coating which is engageable against at least one of the inlet and outlet passages in sealing relationship, and the valve stem has a spring-biasing member outside the chamber for urging the diaphragm away from sealing engagement with the inlet and outlet passages. A peripheral rib is formed in a chamber wall and the diaphragm is held in fixed relationship against the peripheral rib.