I changed both fuel tanks and the sending units with the low volume fuel pumps attached. I removed the fuel reservior and made sure nothing was clogged because I had not used the rear tank. I have one problem, I was running the rear tank and the front tank began to overflow. Could I have the return lines connected incorrectly at the reservior.

 

i suppose that's possible but i'm guessing your fuel reservoir selector switch (that giant oil filter looking think with 2 hoses running to engine and 4 toward the rear of the truck) is screwed up.

 

I was having the same issue with my truck, an 86 5.0 EFI. I believe the part mentioned above is call the "Dual Function Reservoir". They are super hard to find and pretty pricey. I searched around for a few weeks and eventually found one at at Millennium Ford in Burien, WA. The list price was $180, but I paid $108 after a $10 shipping charge. It was worth it though as my tanks no longer overflow. These guys can be reached at (206) 431-7333. That's the number directly to the parts desk. I just called them and they still have 1 in stock. Better get it while it lasts! Good luck.

 

I found one at a junkyard months ago and I think they charged me $15 for it. They didn't really know what it was. Then like a month later before I was about to put it in a brand new one was put on craigslist for $50. score!

the dual function reservoir is a POS design. the 85-86 are a little different than 87+ but an 87+ will work. Just make sure it has 6 ports. if you take one off at a junkyard be careful you don't break any of the port when removing the hoses like I did. I think they are held to the frame with 2 or 3 bolts and I believe they were 10mm.

 

I bought a new fuel reservoir and the problem is gone. No more tank overflow. Thanks guys for the help.