Originally Posted by herk7769
If the vent starts filling after only a few minutes, the pipe may have been installed too low in the tank.
Could just fill the black tank with water. Open the black dump valve and listen for the vent on the roof to start sucking air. If the vent in the tank is above liquid level, as I guess it should be if correctly installed, it should start sucking air immediately after opening the dump valve.
Same thing as sucking the water out of the sink traps when emptying the gray tank.
I bet his tank empties slowly too...