Fuel systems

#1

I have a duel tank 1990 f-150. how come fuel from the back tank feeds into the front tank, and is there any way to stop this from happening?

#2

The 1990 was a return flow fuel system. The valve that switches the return flow from tank 1 to tank 2 is either defective or is not being operated.

#3

The fuel pump in the front tank is bad allowing feedback into it.