Ive read most of the existing threads, but cant find a clear answer. Is there any check valves in the fuel return system on these trucks (95 F350 7.3)? I cant currently use my rear tank otherwise the returning fuel overflows my front tank. I just ordered a fuel selector from Riff Raff diesel, but dont want to put it in if that is not the problem. I can hear the valve switching from under the truck when I flip the switch? Any ideas?