drain valve blacked and not sealing completely

I have the same problem, and after trying to flush/clean/lube them with limited success I just got an extension valve (Valterra T58 twist on style) to stick on the outlet. My wife and daughters have long hair and we tend to get sandy at the beach, and it hangs up in the valve seals. The extension valve doesn't stick out very far and it saves getting brown water on you when you open the cap, plus its cheap and easy to replace without having to get dirty.