Abstract: An upwardly open reservoir adapted for placement in a toilet flush tank is described. The reservoir is designed to be positioned directly below the float of a float actuated flush system. Flush water refill tubes and/or an overflow tube supply flush water separately to the flush tank and the reservoir. Under normal, non-leakage conditions an equilibrium water level is maintained in both the flush tank and the reservoir. Under conditions in which water leaks from the flush tank through the flush valve, the reservoir maintains a flush water level that is independent from the flush tank itself, such that the float is elevated to a level and maintained in a position that prevents a further flow of refill water into the flush tank. A secondary reservoir receives water from the overflow tube and provides water to the main reservoir to raise the float sufficiently to shut off water flow when water supply from the overflow tube stops.