Abstract: A valve, the housing and gate of which are composed of molded rigid polyvinylchloride, the valve body having an interior chamber with a main upstream inlet and a main downstream outlet and an annular valve seat in the housing, and a gate being arranged in the housing which is spring-biased to a normally closed position with an O-ring in sealing relation on the seat to effectively block fluid flow through the valve and said spring being selected so as to have a resiliency to yield in response to predetermined upstream fluid pressure to unseat the valve member and permit flow of fluid through the valve so long as the predetermined pressure is exceeded.