Abstract: A system for relieving excess ground water pressure surrounding a swimming pool is disclosed. A one-way flow valve is inserted in the pool enclosure so as to complete a passageway connecting its opposite sides. The valve comprises an elongated plunger slidingly engaged within a sleeve forming a portion of the passageway. The plunger has a tapered conical seating surface radially diagonal to its long axis. The sleeve contains a compatible conical surface such that when the two surfaces meet by sliding engagement of the plunger, the flow of water through the passageway into the pool is closed.