[DutchmenSport]

If there is 'crud' on the black tank dump valve itself, then there is still something you can try before dismanteling anything. As stated above, block off the end of the dump pipe with another valve, like a Flush King or as mentioned above the Super King. Close this valve and leave the black tank valve open. Now add some water, enough so it will fill the outlet pipe and cover the black tank slider valve. Also add some enzyme eaters, Rid-X, septic tank booster, or something like was mentioned above. Let THAT sit in your pipes, covering the valve and lit the enzyme eaters dissolve whatever is clinging to the valve. Do this first and give it a chance to work so you don't have to tear your trailer apart (yet). Start simple, and work for a more complex solution when the simple does not complete the mission at hand.