dual tank transfer 1994 f150

I am looking for an internal view of a fuel pump or info about it. My forward tank is overflowing and I assume it's a back flow problem from the rear tank. The tank is pressured up and spills when I remove the cap.From what I have read there should be a check valve somewhere to prevent this sort of thing. Question is: where is it? In the pump itself or externally mounted?

Check valve is in the pump. Not really a serviceable thing. You'll need a new pump.

there is also a safety valve on the tank itself as well as a vented cap, you should be noticing a lose of petrol .