1986 f250 5.0 efi 4x manual....

Initially I thought I was having problems with the gas guage or the tank selector switch but it turns out that gas from the rear tank is flowing into the front tank. That explains why (regardless of which tank I'm running on) the gas guage for the front tank always shows it to be full until the rear tank is empty.

So, what do I need to do to determine what's causing this? The rig runs great -- no stalling or starting problems.

I've talked with a few folks about this and they say it may be the fuel selector valve. Some say it's one of the low pressure pumps in one of the tanks.

Can someone tell me how to go about determining what's causing this to happen so I can fix it?

Thanks

Jeff