I've been able to find lots of websites and forums that show how to connect cast iron to pvc in a straight section of pipe, but my cast iron pipes are rusting out right at a pipe union, where a straight pipe connecting into a large pipe in a Y fitting. The fitting Y section seems OK, but the straight pipe is crumbling. Is it possible to fit PVC into the iron Y piece, or do I need to replace the Y piece too and then connect that new PVC Y piece to the remaining cast iron lines some how?