I have a 96 F-150, my rear fuel tank when selected quits, I replaced the fuel pump in tank. same problem, replaced the switch on the dash, same problem. When I turn the key on with switch in the front tank pos. I hear the pump prime. I read talk of a selector valve, where is that? I don't see anything on the rail. I need to get the rear tank working cause the front tank leaks under hold-up strap.