Suddenly I am unable to add gasoline to the rear tank of my 1990 f350, (7.5 liter engine), with a utility body and two gas tanks. When I try to gas up, the gas backs up at the gas cap as if the tank were full. I am able to add gas, but only very slowly, a penny or so at a time. I suspect that air trapped in the tank is preventing the gasolinefrom entering, since I can't think of any more likely cause. How can I solve this problem?

Thanks,
Tom Desmond

When i had that problem, it was because that particular tank was FULL. Steve

are you sure its empty, fuel gauge could be bad. use a screw drive to hold the trap door open and use a flashlight to see if it is full. if not you could have a air lock problem, the vent tube or port could be block with dirt/grime.