The vacuum breaker shouldn't have anything to do with dumping the black tank. I agree with Tobys Home that it must be on the water in side. All the flush valves I have had anything to do with are a line from the flush hookup, to the flush nozzle in the tank. Some of them have an anti-siphon valve, or a check valve in the line to prevent the possibility of the stuff in the black tank getting into the water system. That is the only thing that should be in the line. Check through the access door in the basement and see if you can see a possible problem.
Roger & Cheryl Full Time since 2002, RVing since 1975 2008 Chevy 3500 Crew Cab Dually, 2012 RW 36RL