Abstract: The housing of a pressure-relief valve has an inlet port forming an inlet valve seat, an outlet port, a valve chamber between the ports, and an abutment in the valve chamber. A valve body in this chamber has an inlet end engageable in the inlet valve seat and an outlet end, and is displaceable in the housing between a closed position with its inlet end seated in and blocking the inlet valve seat and an open position engaging the abutment and with its inlet end spaced from the inlet valve seat. A pilot piston in the valve chamber has a throughgoing passage opening at one end at the outlet port and forming at its opposite end an outlet valve seat engageable with the outlet end of the valve body. This piston is itself displaceable in the housing between an open position with its outlet valve seat spaced from the outlet end of the valve body in either position thereof and a closed position with its outlet valve seat engaging and blocked by the outlet end. A spring urges this piston into the closed position.