[Old Dad]

I had purchased and prepared to replace my black tank dump valve a few years ago due to the same problems it sounds like you are having. After dropping the coroplast, I found the rubber in the valve was not aligned properly. I disassembled the valve, greased the rubber and reassembled. It has worked properly since that time.
My valve was leaking as well. I found some trash from original construction packed in the valve seat. This wasn't a "fun" job, but I believe it is easier to clean and grease than replace.
Good luck.