Black tank flush stopped working

I use the blank tank flush about every other dump partly to put 5+ gallons water in the tank so it sloshes around when driving. Couple days ago it would not put any water in the tank so I put 100 psi of air on the hose and still nothing.:(

Fortunately my grey and black tanks are on the same level and unfortunately the same size (what were they thinking) but this lets me transfer grey water to the black when the grey gets full. By leaving the cap on and opening both valves until I can no longer hear water moving I can shift 20+ gallons of grey to the black.
So, yesterday after dumping only the black I put the cap on and opened both valves putting 20 gallons of grey in the black. Drove 100+ miles on curvy back roads and when I hooked up at the next CG... SUCCESS, the black flush works again.:B

Maybe this will work for you if you have this problem.