[bstark]

You have a leak at the black tank flush anti-syphon device. Unless your black tank flush is hooked up permanently to some water source and the shut-off device is leaking there should be no leak at that point as there shouldn't be any residual water pressure there. When you detach/shut-off water supply to the black tank flush, the absence of pressure from the supply side allows a small "poppet" type valve to open which allows air into the system at that "high point" which in turn allows water to drain backward to the flush hook-up which breaks the water column and prevents water being syphoned out of the black tank all the way back to the flush port. there have been cases of the preventer being installed backwards but that would simply not allow water past the preventer to the flush nozzle & into the black tank.