a liquid delivery apparatus includes a vessel having an inlet and an outlet , and a valve between the vessel and the outlet . the valve has an opening , a closure member for closing the opening , and a biasing means to hold the valve in a normally - closed position . a valve control mechanism controls operation of the valve in response to pressure of liquid and air in the vessel . a transmitting means transmits the pressure to the valve control mechanism . the valve control mechanism includes a moveable element that can be acted on by the pressure in the vessel and transmit a resultant force to the closure member to thereby open the valve . the respective effective surface areas of the moveable element and the closure member and the force of the biasing means are chosen such that the closure member is openable when the pressure in the vessel reaches a predetermined level .