Abstract: A flush tank has a downwardly open outlet, a valve for filling the tank with a liquid, and a flush valve having a tube fitted to the outlet and having an upper end vertically displaceable in the tank between an upper position and a lower position. A float fixed to the tube upper end and having an upwardly open compartment has a low buoyancy insufficient to hold the tube upper end above the liquid level when the compartment at least partially filled with water and a high buoyancy capable of holding the tube upper end above a liquid level in the tank when the compartment is empty.