a self - closing valve , having means to discharge a predetermined volume of water for each cycle of operation , is disclosed . the valve includes the arrangement of timing means which determines the elapsed time for its operational cycle , a one - way valve means to expel air from the metering chamber during the actuating cycle of the valve , and air metering means which replaces air entering the mixing chamber at a constant uniform rate , all of which cooperate with the valving means to discharge a predetermined volume of water .