All flush tanks shall be equipped with an antisiphon ball cock conforming to ASSE 1002 or CSA CAN/CSA B125. The ball cock backflow preventer shall be located at least 1 inch (25 mm) above the full opening of the overflow pipe. A sheathed ball cock shall be installed in all gravity flush tanks in which the flush valve seat is located less than 1 inch (25 mm) above the flood level rim of the bowl.