A valve for controlling the level of liquid within a chamber (10) is disclosed. The valve comprises a liquid inlet (30) and a primary float (40) that is movable in response to a change in the level of liquid (20) within the chamber (10) between an open configuration in which liquid (20) is able to flow through the liquid inlet (30) into the chamber (10) when the liquid (20) within the chamber (10) is below a predetermined acceptable level, and a closed configuration in which liquid (20) is prevented from flowing through the liquid inlet (30) into the chamber (10) when the liquid within the chamber (10) is at or above the predetermined acceptable level. The valve also includes a secondary float (50) that is movable in response to an increase in the level of liquid within the chamber (10) above the predetermined acceptable level from an inoperative configuration to an operative configuration in which the secondary float (50) imparts a force upon the primary float (40) that urges the primary float (40) towards its closed configuration.