Progress: I finally had a little time yesterday/today to mess around with the 7.3 again.
I installed clear tubing between the top of IP and the main return line pipe. I already have another clear tube between #1 injector and the fuel pump housing.
After running the engine for a while and shutting off, after a few minutes I start to see bubbles come up to the top of IP. And eventually also some up to the fuel pump housing.
Ok, not earth shattering. I was expecting this. The problem has been that with the banks turbo installed I can't get to the very back of the engine to inspect the return line tee(s). But after removing air filter housing I was able to shine a light back there and saw enough oil (diesel?) grime to suspect a small leak. It's still not possible to get a good view or put a wrench on anything though.
Also, the return line from passenger side injector closest to firewall goes down into the valley beneath the turbo and disappears. I believe there must be a *second* tee down there into the main return line, but can't even see it. super frustrating.
So, I got a bright idea. Or rather two.
1. To confirm the leak location, I cut the return line under the truck and put a plug in it. Then I removed the line at top of IP and blew compressed air (20 psi) through it. Sure enough, I could hear air coming out where I suspected and see it blowing small debris back there. bam! problem identified, after all this time.
2. I'm going to bypass the factory return lines which are impossible to work with. I will simply run hose from the IP top (return) to the return line at bottom of truck, and tee the front driver side injector into it. Then I will route together the returns from the two injectors closest to the firewall so that the passenger side injectors have a return path. This puts everything up where it is easy to work on it and identify any possible leak in the future. And I only need to change out one injector cap.
Ok, so that's the plan for tmw. Hopefully my hard start problems will soon be a thing of the past and the return line system will be cake to troubleshoot and work on.
Anyway, I hope this helps someone down the road. I haven't really seen anyone talking about the return tee(s) at back of engine being the problem, or hard to access because of turbo, or plugging the return line and using compressed air to find the leak.
The return lines on this engine are conceptually simple, but impossible to access in practice without removing the turbo entirely which makes it a big PITA. It's high time for simplification.
edit: I also removed the outlet at top of IP pump because I learned there is an o-ring inside there that sometimes goes bad. My o-ring looked perfect, but I replaced it anyway, and used thread sealant when replacing the two threaded pieces.
edit:
here is a video of another truck with a banks turbo that also has leak in about the same location as mine. But the leak in the video is much worse than mine, so it is obvious.