Clearing the black-water dump valve

I, too, back flush with the grey water but I do it all with the pressure in the full grey tank. I begin by dumping the black only when the grey is full. I dump the black first and, after it is empty, I pull the handle on the grey which allows the grey to rush up into the black. Even though some runs out the slinky, of course, there is plenty enough pressure to shoot straight across into the open black tank. After about 5 to 8 seconds, I close the grey valve and watch the water (and more) run back out of the black tank. When empty, repeat. With a full grey tank, I can do this 4 or 5 times and still have plenty of grey left to run through the slinky to flush it. I use a clear elbow between the outlet and the slinky to see what's going through and it didn't take much practice to get a feel for how much to successfully flush the black as well as when to stop the back flushing because the grey was beginning to run out. If you do this, you will see lots of black sediment and some paper run out and, with each successive cycle, you will run more and more clear. Noticing how much comes out on the first back flush, you will wonder how that black valve didn't get jammed each and every time (prior to using this method). One final tidbit... I close the black tank valve before the last grey flush runs out thereby trapping some grey water in the black tank. By the way... I do have the on-board black tank flush but, using the above method, I've found it's just not necessary.