Abstract: An overpressure-control valve has a valve housing formed with a front output chamber, a back intake chamber adjacent and back of the output chamber, and a valve seat lying between the chambers. A valve body can move in the housing forward toward the valve seat into a closing position on the seat and blocking flow between the chambers and backward from the valve seat and into an open position permitting flow between the chambers. This body has at least one forwardly directed face and at least one backwardly directed face exposed in the chambers. The effective surface area of the forwardly directed face is greater than that of the backwardly directed face. A spring urges the valve forward into the open position with a predetermined generally constant biasing force so that the valve body moves back against the spring into the closed position when the backwardly effective pressure on the forward face overcomes both the force of the spring and the pressure on the back face.