Abstract: A device for use in a water cabinet possessing a valve which stays open until the reservoir is empty. An auxiliary reservoir (20) without bottom is placed within the main reservoir. At the base (27) is a slit opening permitting liquid communication from the base of the main reservoir. At the top of the auxiliary little reservoir is located a check valve (24) controled by a lever arm (18) which at the same time acts as the flushing arm of the toilet cabinet; the check valve (24) closes as soon as is released the lever arm (18). When the little reservoir is full and its valve is closed, even when the main reservoir is being emptied the little reservoir remains full while the main reservoir evacuates, thereby economizing a quantity of water equivalent to its volume. The lower end of the auxiliary reservoir (20) may be cut for fitting in a given water cabinet reservoir.