[BB_TX]

The black tank flush line drains from the point of the vacuum breaker valve to the tank. But I don't think it can drain from the line from the vacuum breaker valve back to the external connection. The valve is located in a water line that goes up in a wall, connects to the valve, then down to the tank. When the flush line is pressurized the valve actuates to prevent water leaking out and air from leaking in. When the flush water is turned off the valve opens to air allowing air to enter the line and prevent siphoning either direction. That would leave water in the line between the external connection and that valve.
I could be wrong (wouldn't be the first time) but after working on mine that is certainly the way it looked.