I got this truck a little over a year ago and we did a bed-off restoration. We dropped the fuel tanks and everything on the frame and painted the frame. We also put headers on the engine because the original manifold was cracked. In order to do this we had to delete all of the emissions. When we put it back together one of the inlets on the fuel tank selector valve broke off. We bought a new one and mounted it to the frame. After we started running it for a while and made sure everything in the motor was good we filled both tanks up completely. When we were driving it on the front tank down to the muffler shop to get the new exhaust installed gas started pouring out the gas cap of the rear tank. We stopped, took the cap off the rear tank, it was overfilled and extremely pressurized. We came to the conclusion that fuel was being sucked from the front tank to the back tank. After we got it back from the muffler shop, we got another selector valve thinking that the one we order was just a bad one. After about one week, the same thing happened again. So we know when we drive on the front tank gas goes from the front into the back and when we drive on the back tank gas goes from the back to front. We tried switching the return lines and it just transfers faster. This was one of the first years for EFI so everything is a little weird. A fuel line and a return line come out of both tanks and connect at the switchover valve. From the switchover valve one fuel line and one return line run to a high pressure pump and then to the engine. There are no electronics coming off the switchover valve. It runs off vaccum from the tanks. Also, coming out of both tanks is a breather line that connects behind the cab and then runs into the engine bay. The breather originally went to the emmisions stuff and now it is just an open line in the engine bay. We took it to two mechanics and watched several Youtube videos trying to fix the problem but it still occurs. Is there a simple fix for this problem or do I just have to deal with it.