I have a drum trap attached to the drain beneath my bath tub. It's easy to get to in the basement. Never had any problems with it, but the cap is really corroded - it has about a quarter inch of green gunk all over it. I'm going to be replacing the waste line from the tub soon, so I was wondering if it would be best to leave the drum trap as is and maybe replace the cap, or should I replace the whole thing with a P trap? Any advice? Thanks!

Convert to PVC and install a "P" trap. You can't get a snake through a drum trap. We don't use them any more. Good luck, Tom