My cellasr drain is made of cast iron and is about 35 years old. It is connected to a perforated plastic pipe that is installed around the foundation footings and also under the cement cellar floor. On occasion a stron rain storm will fill this pipe faster than it can drain which causes the water to backflow into the cellar. The cast iron drain has a 6.5" cover with perforated holes in a circle around it. It has a 2'5" inside diameter drain hole dropping into the plastic pipe. I can not find a back flow preventer with this dimension. Is there a solution to this, or will I have to break this drain out and install a new plastic one with a 2" or 3" hole? Any suggestions will be appreciated as to what to do and where to purchase it.