What is the best way to remove stains from plastic plumbing?

There are some urinals that need to be cleaned. They have plastic plumbing. there are some leaks and some caulking to fix the leaks. The pipe and the caulk are discolored. If we scrub the caulk with enough force to remove the stains we would also remove the caulk and reopen the leaks. The plastic is also discolored, cleaning it is a very temporary solution as overflow, condensation, leaks or plain bad aim has them discolored again in short order. Any tips would be he Any tips would be helpful.