Gas cap leaking?

I'm having this issue with my truck. When I fill up the back tank and start driving away, it will push the gas up through the front tank's gas cap and spill out everywhere while driving. I thought it might be the vent valves and to much pressure but that wasn't it. Where should I go from here? I was thinking the tank switch reservoir thing that is next to the inline pump and filter. Or if not that, maybe the mechanical fuel pumps on the actual tanks? Honestly, I have no idea, anything would help out thanks.