Abstract: A toilet tank flush valve for use in the demand mode, and which includes a discharge control and closure valve for controlling the flow of water from the tank or reservoir to the bowl, and with the valve further having a generally cylindrical configuration with an open top and a closed bottom defining a water retaining chamber, with the chamber extending upwardly above the discharge drain opening and being designed to retain a fill of a certain first quantity of water. Pivotal opening of the valve, while tipping a portion of the water outwardly of the enclosure, will not discharge more than about 70% of the volume of water retained within the enclosure so as to permit a more positive seal of the valve onto the discharge drain opening.