On thanksgiving, my wife and I drove to her grandparents' house for dinner, about 20 miles away. On the way there, the truck ran outta gas on the freeway. This surprised me a little, as the PO told me that the truck automatically switched and drew gas from the rear tank, when the front tank was empty. I knew I put 25 gallons of gas into the truck, and expected to get over 400 miles out of it, and had only gone 150 miles. Plus, the fuel gauge was inoperative.
I had a 2-gal gas can in the back of the cab (the PO had left it there, when he admitted the fuel gauge didn't work) full of fuel, so I added that to the tank, and filled up at the next gas station. (Refilled the gas can, too, just in case...) I filled the main tank, and topped off the rear tank, but the rear tank only took 1 gal. We finished the drive to her grandparents', and when we arrived, I saw fuel dripping out on the ground. It wasn't a fast leak, so I figured it must have just been a little bit of overflow or something.
2 hours later, after dinner, the drip was still there, running just as quickly as before.
When I arrived home, I figured I had better double-check the fuel supply system.
Surprise, surprise, the PO was full of crap. There was no selector valve for dual fuel tanks, the main tank was hooked directly to the supply lines, the wires to control the selector valve were just tucked away in the frame, and the ends of the wires for the fuel level sender and fuel gauge were loosely
twisted together.
I decided to pull the bed off and get to the tanks (lots easier than dropping the tanks). After pulling the plastic bedliner, I determined that some crack-monkey PO had welded something to the bed screws, then later cut/ground it off. I got 5 of the screws out, but the torx head on one screw was too badly damaged to get a grip on. I ground the top of the screw flat, and welded a 5/8 nut to the top, then removed it with a ratchet.
I pulled the pump/sender from the main tank, and determined the sender was bad. I pulled the pump/sender from the rear tank, and got another surprise: the tank was nearly empty. I had filled it just a couple of hours ago!
Turns out the lines from the rear tank just hung open under the frame, where the selector valve
should have been. When I filled the rear tank, it started siphoning the gas out, slowly, and drained $30 worth of gas onto the highway and grandpa's field.
The next day I got a selector valve, a switch, and a pump/sender assembly for the main tank. I cut the connectors to the main tank, and hooked an octopus of hoses and wire up to the valve. I went to test the switch, and when I put it in the "Aux" position, all the lights in the cab went out. I determined that the main fuel pump had been hotwired, and when I tried to switch it to ground, it shorted the battery. I poked around with the wiring a bit, pulled a vamp clip, removed the
other hotwire that bypassed the fuel pump relay and the inertia switch, and got that problem cleared up. (Stupid freaking crack-monkeys!)
Plugged in the switch again, and determined that the switch was a momentary switch...it switched the selector valve, and supplied power the the corresponding fuel pump, only when it was
held to one side or the other. It was the one that was
supposed to go with the selector valve! (Stupid freaking Auto-zone parts!) I found a DPDT switch in my junk box, and put that in the circuit instead of the useless momentary switch.
I installed the pump/sender in the main tank, tested everything, and it appeared to be working. I drove a couple miles that night without any problem. The next day I left for work, and about a mile from home, I ran outta gas again!
The fuel gauge indicated the main tank was full, the switch was in the "Main" position, and I knew I had at least 13 gallons of gas in that tank!
On the side of the highway, I dove under the truck with a flashlight and looked at the fuel hoses. I had used some clear tygon to hook the ends of the supply lines to the selector valve, so I could see what was happening. The line from the main tank was full of fuel, but the line to the high-pressure pump was empty, as was the line from the aux tank.
After some intense thinking, I suspected the valve was plumbed backwards...so that the main lines were connected to the high-pressure pump when the auxiliary pump was running, and the aux lines were connected when the main pump was running. I set the switch to "Aux" to hook up the
main lines to the high-pressure pump, then crawled under the truck again and disconnected the selector valve wiring (so it would stay where it was) then set the switch to "Main" to power the pump in the main fuel tank. It worked, and I got to work.
On my lunch break at work, I dug out the instructions for the selector valve, and verified that I had indeed hooked it up according to their directions...they had simply drawn the diagram wrong! I again crawled under the truck to switch the lines...the supply lines and the return lines both.
On my way home, I noticed my fuel gauge dropping awfully fast...and after about 12 miles, ran outta gas a half-mile from home (near a gas station, luckily). I was tired of paying for gas, so I dumped my gas-can into the main tank and finished the drive home. After dinner, I crawled under the truck and inspected the return lines; after all that, I had hooked up the return lines backwards again! It was drawing fuel from the main tank, and returning it to the rear tank! I tried to set the switch to "Aux" and pump the fuel
back into the main tank, but apparently the rear fuel pump wasn't working well (I suspect it is shot...it pumped weakly when I tested it, but probably not enough to run well). I corrected the lines again, and bought more gas...and the rear tank was nearly full, just as I had suspected.
So after all these mishaps, I need to pull the bed off again, and replace the pump in the rear tank. I still have the old pump from the front tank, that works well, I removed it when I replaced the fuel sender unit.
Again, no pics, sorry.
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I decided to fix the warning lights on the dash...or at least the brake lamp.
A little tinkering showed that the switch and vacuum line had nothing to do with each other, but some crack-monkey PO had stuck the line over the actuator for the switch, which had broken off the mount behind the parking brake pedal. The vacuum line had no suction on it, either. 
Also used one to keep my door shut... the drivers door too
