Abstract: A plate (42) is mounted to the outer flange (12) of a standpipe (10). A valve element (96) and a float (68) are mounted on the plate (42) to pivot about fixed axes A.sub.1, A.sub.2, respectively. A toggle link (90) extends between the valve element (96) and the float (68) and is connected thereto at pivot points P.sub.1, P.sub.2, respectively. When the float (68) moves into its down position in which the valve element (96) is in an open position, the float (68) passes over a center line C defined by the fixed axis A.sub.2 and the pivot point P.sub.1. This overcenter movement of the float (68) prevents the valve element (96) from moving into a closed position unless the float (68) is moved upwardly by a rising liquid level.