There should be a hole in the return line that will prevent all the water from flowing back into the w/d... You need to test this and get the water level right in your display tank.
You should get the gravity flow running and shut the pump off.
The water level should not overflow your wet/dry... If it does you'll have to take out more water.
You can adjust your waterlevel in the display tank and the wet / try by raising and lowering the collection (tankside) container of your overflow system.
I can help by email if you need more help..
kesselj@greenapple.com