A device for controlling the filling of a cistern in a main reservoir of a toilet flushing system includes a water inlet controlled by a valve. The closing of the valve is controlled by a float that is movable from a high position to a low position. The float is contained in a float housing in the high position. The float housing connects to the main reservoir by at least one siphon having two tubes: a suction tube linked to the float housing; and a discharge tube being linked to the main reservoir and opening into an auxiliary reservoir. There is a control mechanism provided with a mechanism for priming the siphon, wherein shifts between an initial position and an actuated position of the control mechanism result in overpressure or underpressure at the siphon so as to control draining of the float housing.