A pressure-relief valve has a housing forming a chamber extending along an axis and a valve seat centered on the axis and subdividing the chamber into an outer high-pressure compartment and an inner low-pressure compartment. The seat defines generally the only fluid-communication path between the compartments. A pump and a load can be connected to the high-pressure compartment and a sump to the low-pressure compartment. A valve body is formed with a central radially outwardly open main groove, an outer face exposed in the high-pressure compartment, an inner face exposed in the low-pressure compartment, a radially outwardly open control groove offset axially inward from the main groove toward the inner face, and a diametrally throughgoing control bore at the main and control grooves. The body is axially displaceable through the seat between a closed position substantially blocking the seat with the main groove and control bore axially offset therefrom and an open position axially offset toward the low-pressure compartment from the closed position and with the main groove and control bore at the seat for flow through the main groove and seat between the compartments, and through an intermediate position between the open and closed positions and with the control bore at the seat for flow through the control bore and control groove between the compartments. A spring urges the valve body axially outward toward the high-pressure compartment into the closed position.