What kind of washer are you using between the steel pipe and plastic pipe? Is it a nylon washer with beveled edge or like a very thick rubber band? You should have a galvanized pipe (nipple) coming out of the cast iron to which you insert the plastic with some type of washer and nut around the plastic. I've seen all kinds of washers used but usually it's the rubber band looking washer and frequently people wrap it with a packing type of string (can't remember the correct name here) if it leaks. The nylon washers work pretty good but IMHO the others are better in this situation. Be sure the nut is not cross threaded on the pipe nipple.