A flow-control valve assembly comprises a housing formed with a valve chamber, an inlet port and an outlet port both opening into the valve chamber, a valve seat at one of the ports, and a control chamber. A valve body is displaceable in the valve chamber between a closed end position engaging the seat and blocking flow between the ports and an open end position clear of the seat and permitting flow between the ports through the chamber. This valve body has a valve face exposed to the pressure at the inlet seat in the closed position. A control body in the control chamber has a control face and an oppositely directed counterpressure face. This control body subdivides the control chamber into a control compartment at the control face and a counterpressure compartment at the counterpressure face. The counterpressure face is of greater effective surface area than the valve face. A link interconnects the control and valve bodies for joint movement for increasing the volume of one of the compartments while decreasing that of the other compartment as the valve body moves from the open to the closed position and for decreasing the volume of the other compartment while increasing that of the one compartment as the valve body moves from the closed to the open position.