My Excella has the "won't close 100%" problem. Liquids leak out, solids build up, the black tank will not empty completely. It takes half an hour running the rinse system to flush out the tank till it runs clear, but the dump valve still will not seal.
I can hardly wait to start working on the dump valve.
I've done it twice on my other trailers, so I know how "rewarding" it can be.