One-way elastomer valve

A low-profile one-way valve assembly that attaches to a liquid conduit. A valve fitting supports a recessed, bi-leaflet, elastomer valve piece. A compound curvature at a valve base, raised seal forming surfaces at provided leaflets, tapered extension surfaces at the leaflets and a tailored sidewall thickness profile at the leaflets assures a durable watertight sealing action at the flexible valve piece. An O'ring mounted to a threaded fitting surface seals the assembly relative to a supporting conduit. A cover is also provided to protect the valve piece from external damage and significant back pressures.