I've had cracks and leaks developing in my old fuel lines, so about a month ago I dropped my passenger side tank to replace the sending unit and also replace all the lines with new SAE 30R9. The SAE 30R9 fuel line is listed for fuel injected engine, but the key feature is that it holds up much better to the new fuel with ethanol. In my case I needed 3/8" (fuel feed), 5/16" (vent) and 1/4" (return). SAE 30R7 line would also work, but is is not rated for as much pressure. I also used fuel injection hose clamps, since they are smooth and don't tear up the outside of the hose like worm clamps.
- First drain the tank, or run it as dry as possible.
- Remove the worm clamps where the tank lines connect to the steel lines or to the switching solenoid, and make sure the lines are free of the connections.
- Also take out the bolt for the clamp that holds the three lines together and clamps them to the top of the frame.
- Remove the tank ground wire from the bolt that holds it to the frame. Pop apart the electrical connector for the sending unit wire. Both should be inside the frame and fairly easy to reach.
- Loosen the 8 bolts that hold the tank brackets, and leave the tank just hanging.
- Remove the gas cap and take out the two screws/bolts that hold the filler cap bracket to the truck bed.
- Put a low profile hydraulic jack under the tank and put a wide board (6") between it and the tank to spread out the load. Get it up just high enough to feel it move the tank up and down a little.
- Take the 8 mounting bracket bolts completely out, and then start to lower the jack.
- Guide the fuel lines up and over the frame as the tank goes down. Guide the two wires through the hole in the frame. Also watch out for the gas filler bracket catching on the bed. Its nice to have a helper watching.
- At some point you might not be able to get the jack low enough and you may have to slide the tank off the jack to get it out. It barely fit under my K25 with enough clearance to pull out the tank.
The new sending unit will come with a retaining ring and gasket. To get the old ring off you should tap on the ring tab with a brass punch (no sparks) to turn it counterclockwise. I did not have a brass punch, so I aired out the tank, covered all the fuel openings, wet down the top of the tank, and used a regular punch. I also took a picture of how the sending was oriented so that I got it right when I put the new one back in. The tanks are universal left/right, but sending unit and lines go in opposite directions for each tank.
Drain and clean the tank. Mine was pretty clean, considering that its been on there since 1975. Install the new sending unit, gasket and retaining ring. Make sure it has a ground wire (it comes on the new sending unit), and move the wire for the sending unit over from the old sending unit. The sending unit wire just pops onto a threaded post in the sending unit, but mine had been on so long that it really did not want to come off the old one.
The sending unit rubber lines do not need to be very long (10"), but I left them quite a bit longer when I installed the tank, and then cut them to lengths that matched up well with the hard lines. The main reason was that the 30R9 rubber hose is expensive and I didn't want to waste any. Make sure you also guide the two wires to an area where you can reach them and pull them down to make the connections.
I still have one more tank to drop and replace sending unit and lines.
FYI - I think the rear 25 gallon tanks may have been an option for some chassis-cab trucks that used the pickup frame; maybe for commercial versions like box trucks. AFAIK Subs used the 31 gallon or 40 gallon tanks.
Bruce
