Abstract: A combination lock and relief valve in which a single valve poppet provides both functions. The combination valve is so arranged that the opening pressure of the poppet as a relief valve is independent of system pressure and remains constant regardless of system pressure variations. Moreover, when the poppet opens as a relief valve while the system pressure is on, the differential pressure across the poppet is relatively small whereby to minimize or eliminate erosion of the valve seat as occurs more readily when relief valve poppets are opened with high pressure differentials thereacross.