Whenever I see a thread about black tank flush problems, I think back to the thread, "Black Tank Flush" started by JmesDean in 2006
When he attached the water hose and turned on the water, he heard a "pop". He thought something popped off and reduced his water pressure. One possibility is that if you attach a hose and turn on the water with too high pressure you might damage the check valve or something else in this "No-fuss flushing system".
So whenever I attach a water hose to my black tank flushing system, I feel the hose as I gradually increase the water pressure to the point where I can see that it is adequately flushing the tank without blasting out the system.