My setup was on a 1993 7.3 factory turbo. The concepts and methods will apply to any mechanically fuel injected vehicle, whether an old Volvo gasser sedan, a Rabbit Diesel, or N/A 6.9/7.3.
1) Clear vinyl tubing, obtained from your local hardware store. Food grade vinyl is fine, this is a temporary set-up. If you want permanent clear line, get fuel rated or bio-fuel rated inexpensively at McMaster-Carr. My size is 1/4" ID.
2) hose clamps, 7/16".
For my test, I connected:
a) from my electric lift pump to the filter housing
b) from the filter housing to the #1 return cap
c) from the IP to #2 return cap
d) from the #8 cap to the tank return.
Make sure the vinyl is pushed all the way on and the clamps are tight, or you *will* get air leaks back from the vinyl connections which kills any chance of this test helping you.
Links to the pictures are here, descriptions are below and on the photo site.
Clear Line Test pictures by Michael_Phillips_uhhuh - Photobucket
Clear Line Test Filter to #1
Description: Line from filter housing to cap #1. It is looped to catch air bubbles, which allows one to view the travel of bubbles to identify the source of bubbles. The fitting on the filter header contains a check valve, which is a small glass or hard plastic ball and a tiny spring. The ball is against the housing, the spring is in the barbed fitting. This prevents fuel from traveling back to the tanks from the filter side, and allows excess fuel to run from the filter through the return lines back to the tank while running. This design also allows an electric lift pump to push air out of the return lines before I start the truck.
If one sees air from the filter housing there are several leak areas to check, including a bad check valve, holes in the fuel hard lines from the tank, seals at the housing for the fuel heater, filter, or drain. You can see a big air bubble at the top. That bubble didn't travel or expand during the test, so I consider it a pass. (Keep in mind that replacing a line, even to do this test, introduces some air into the system at first.)
Fuel Clear line #8 to tank return
Description: This hose is running from #8 to the tank. Air coming up from the line behind the block (tank return) indicates possible damage to the hard line running to the tank. Air coming from the #8 cap indicates a leak at a cap.
Clear Line Test Pump to Filter housing
Description: This was a new pump installation, so I wanted to verify my lines from the tank were good, in addition to verifying the pump was good. Notice the big loop to trap any air bubbles, again.
Clear Line Test Filter to #1
This is showing a lot of air bubbles. In this case, the bubbles were because I had just installed the clear lines. In reality, there were no air leaks at the pump or filter housing.
Clear Line Test IP to #2 cap
Description: This verifies the pump seals aren't leaking, and also verifies the pump hard line from the filter to the pump isn't leaking. Air coming from the #2 cap, once again, indicates a leak at the cap. The first cap is gray in color, and is a factory cap. It's dull, dry. The second cap (behind it) is black and is a new cap off ebay. It is wet around the top, indicating a fuel leak. Air will get in there for sure. Don't try to cheap out on return line/cap kits. I ended up buying ford o-rings and caps after this cheap cap failed so badly. I have heard there are several better places to get the kits, but I needed to fix this right away. There is a huge air bubble from the pump side, this went away after a while of running, and was due to the swap to clear lines for this test. What one is looking for is NEW air being introduced after getting rid of or monitoring existing air in the lines.
I hope this helps.