I recently bought a 1987 f150 with dual fuel tanks and gas keeps switching tanks (pumps from rear to front
vice-versa and then overflows out the fill spout on to the ground)and gas will flow from front tank to motor and not rear, or it switches and draws from the rear and won't from the front.

I already put new fuel pumps in both tanks and the fuel pump on the in line and a new fuel cell vacuum unit and fuel relay and new fuel selector switch.

OK yes i have had it.
What is wrong with this fuel system?????

 

Yep, they had a recall. I got my 91 taken care of last year, but the recall will only effect your truck if was under 11 years old. I got mine in under the wire. I got a new rear fuel tank and sending unit to boot. I hate to break the news to you but, I bet they turn you down. You might ask the dealer though. The fix is stainless check valves that install inline between the fuel pump and the fuel line at the nipple on the top of the sending unit. I think there are check valves in the pumps but they quit sealing with age. I don't know what the repair valves would cost if you had to buy them. They might not be too bad, they usually sell parts to fix their mistakes cheaper.
I don't think it would be possible to even rig up a fix to get you going again. No matter which tank I would run on the other would pump to the other tank, they never overflowed, so mine never "cross flowed" too much.

 

the problem with the recall is it only covers 150,000 miles or X 3 of years /my 92 was out of miles but I knew a girl at the dealership and she got it fixed for me . Its the valve mine was pumping out of the back into the front .

 

There's not a cross flow recall for '87 year trucks. It was for the newer trucks with high pressure in tank pumps only. I would check the selector valve on the frame. It is operated by pressure from the in-tank pumps, so it could be stuck one way or the other.