Not getting fuel. told not to use the rear tank

I had an 86 F150 with dual tanks, it had a similar problem. The problem ended up being the switcher valve (switches from one fuel tank to the other). Mine was electrically controlled and would switch from rear tank to front tank, but (front tank also had a leak) would not switch back to rear tank. I found out that my switcher valve (after I tried rebuilding the switcher valve myself, and determined that it was not repairable and needed replacement) was defective. They are also not cheap-about 10 years ago it was about $150! Don't know if yours is the same model, but I found the easiest thing for me to do then was remove the front tank, since it leaked (metal mounting straps worn through in spot where insulation came out sideways), and remove the switcher valve. I always left my tank switch set to REAR (dont know if anything would have happened if I had moved it to FRONT anyway, with valve and tank gone) and it always worked fine from then on out.

What you need to do is probably a 2 man job: Have someone inside the cab have ignition on and work the switch from one tank to the other, while you are underneath the truck at the switcher valve. Your job would be to determine if the valve actually works both ways, or just one way, or not at all.

Also there are fuel filters in several places: there is one in the switcher valve, further up along the frame toward the engine, and should be one somewhere on the engine, too. Also each tank should have an in tank pump. Several places to check, it's a big job, its messy, gasoline is toxic, not to mention explosive. Wish I had better news for you.

Hope I've been helpful.