I too recently had a problematic black tank drain valve. It would not dump last month, but I could hear the valve motor working for both the open and close functions. The repairman I consulted with said to first try a 32 ounce bottle of RV Digest It - $12 - let it work for at least 24 hours and see if the tank would dump. It did. While reading the website of the drain valve manufacturer it said the valve motor, when opening and closing, should "buzz" for about 1 1/2 seconds. After playing with both the black and grey tanks' dump valves I noticed the gray valve's motor would buzz for the 1 1/2 seconds, but the black tank valve buzzed for only a half second to open. It turned out the black tank valve was opening only about 1/3. I put an Allen wrench tool in the manual fitting and opened the valve manually... which it did. When I closed the valve the motor would buzz for only an instant, thus was not closing properly from full open. I chatted with the drain valve lady at the manufacturers help line who told me those drain valves have about an eight year life. So my valves, assuming they are the originals, were well past there expected life. I replaced the black tank drain valve - $108 including freight. I probably should have replaced the gray tank valve while in there! Replacing the valves is easy, but getting the water bay aluminum plate off and getting the water manifold off its backside and replacing the manifold is a real PITA! Of course, after getting it done two of the electrical switches would not work - the fresh water hose reel and the fresh water tank drain valve. Pulling the aluminum plate away far enough pulled on and unplugged a couple wires. I had to get a guy to come and figure that out - took him about 15 minutes after I spent two hours unsuccessfully. All seems well now.
