outer periphery to receive an annular o ring or sealing ring 25 which is adapted to seat on the annular seat 19 the mean diameter of annular ring 25 determines the pressure area which is acted upon by the pressure within the raft chamber acting through bore 20 the piston 22 is centrally recessed as at 27 to provide a seat for one end of a compression coil spring 30 the upper inner peripheral edge portion of cylindrical housing 11 is recessed as at 31 to receive an annular cap 32 which cap 32 engages the other end of compression spring 30 cap 32 has a central bore 33 the recessed portion 31 is further grooved as at 35 to receive a retaining ring 36 which frictionally engages annular cap 32 and holds such cap in position thereon the lower portion of cylindrical valve housing 10 has a plurality of circumferentially spaced bores 37 interconnecting the central bore 18 with atmosphere the area of the bottom surface of piston 22 is the effective pressure area which maintains the piston valve 22 open in the operation of the pressure relief valve described assume the raft to which the valve is connected is pressurized to a pressure less than the pressure which actuates the pressure relief valve assuming a pressure of 4 0 psig for such actuation where the pressure rises above this pressure rapidly then the piston 22 is moved upwardly compressing spring 30 and unseating o ring 25 away from.