Abstract: A one directional flow valve assembly for a fluid system. The valve assembly is comprised of a valve body having an internal valve chamber in communication with the ambient environment with a valve seat of a predetermined configuration. The assembly further has a cap structure with means to position and align a sealing membrane with respect to the valve seat. A flexible elastomeric sealing diaphragm is provided which is under tension when the diaphragm is in a sealing configuration. A carrier plate structure may be provided to position the sealing diaphragm. The sealing member is in tension adjacent to the sealing seat when in a sealing position.