Abstract: A pressure relief and vacuum vent valve includes a valve body defining a cylinder having an inwardly facing valve seat. A valve piston is positioned in the cylinder and includes an annular closure portion axially movable toward and away from the valve seat to close and open the valve. The piston has an axial bore. A spring, of considerable rate, urges the valve piston closure portion toward the valve seat. The piston is selectively movable axially against the spring to open the valve and release pressure from within the valve body. A flexible duckbill vent valve is integral with the valve piston within the bore and opens inwardly for venting fluid therethrough only in response to vacuum within the valve body.