A nozzle for use in controlling the flow of liquids incorporating a positive pressure generating device which provides a positive pressure to a diaphragm chamber which serves to retain the valve in an open position after manual opening of the valve by means of a manually operable actuator mechanism. The valve actuator mechanism includes closure means for moving the valve closure means to a closed position and releasable opening means for moving the valve closure means to an open position. A releasable opening means is released when the diaphragm chamber is positively pressurized at a pressure sufficient to retain the valve closure means in a predetermined open position. This provides a nozzle which, after initial manual opening, will be automatically closed when the positive pressure provided by the positive pressure generating device is cut off by the rising level of liquid within the tank. The valve may also be closed at any time by releasing the control handle and permitting it to return to its closed position.