Abstract: A pressurized-tank water flushing device has a hydraulically controlled flushing valve operated by a control valve for selectively applying a pressure differential to an actuating piston, one face of which is permanently subjected to the tank pressure, and the other face either to the water supply, or to atmospheric pressure. The actuating piston has a hollow shank acting as a slide valve which switches on the supply of water to the tank when the flushing valve closes, and cuts off the supply of water to the tank when the flushing valve opens.