Air in fuel issue

I've been having an air in fuel issue for a while now and I am perplexed as to what it could be. Here's what I have, have tried:

Truck is a 2016 389, isx
L tank 90 gal R tank 135

This issue started in the middle of my produce season back in July. I do a run daily I use about 50-60g per run. The first time it died on me it had just under 3/4 fuel. I primed it, limped it to a truck stop, scratchd my head for a while, filled left tank, same problem, then filled right tank, problem resolved. Both tanks were approximately same fuel level. When I got to my home-fix, I put new supply fuel lines from tanks to T and from T to davco. *Princess auto did not have 5/8", so I took 1/2" lines, with 5/8" jic connectors. Same problem. Put taps on the return lines of both tanks to try to isolate problem. Cut R tank suction side off, no air in fuel issue. So I switched the lines so the return is now the suction and vice versa. That worked all summer, until now. I was able to run like normal, down as low as I dare running on R tank only worked fine. I guess the return line goes all the way to the bottom (figured it must have to be almost as long as main pipe). Now it is doing the same thing. Air bubbles coming up to the davco, over a few minutes fuel level going so low (at idle) it will die. At throttle it will die faster.

So what I know now is:
Running on L tank only, R tank both taps or just supply tap turned off, truck runs as normal.
Open supply on R tank at just under 3/4, air bubbles will come and kill the truck.
On the road, it will not empty the L tank, they stay the same level.
Today I disconnected the T valve unsure if it has a flapper in it - it does not.
Take T out of equation completely. Line from davco to new connector to new fuel line and drop that into jerry can of fuel = no air bubbles. Connect to originally the return and air bubble issue comes back. I have the truck in the driveway with the just under 3/4 where it begins to act up. I am running these tests with the fuel cap off. So the tank vent should be no issue since the problem persists. All I can come up with, considering if connected to either supply or return and it draws bubbles I originally thought a pinhole a few inches down in the supply tube. At the exact same fuel level now same problem its pulling from return. Running on L tank and the top 25% or so of R tank for winter playing chicken would really suck. The pickup and supply pipes are welded in to the top of the tank. I have anti-siphon device (tube with holes) welded into the tanks from factory. Is it possible for both tubes going into the tank to have a pinhole at exactly the same height, or is there something I am missing here?

Thanks

 

I think I've ruled out the davco because of a few tests I've run. First was my own homemade clear line just from davco straight into a pail of fuel, no bubbles. Now I have basicly run the same test except using the fuel lines and again into pail of fuel - no bubbles. If I connect it to R tank and close the tap, no bubbles. Open tap, bubbles. Problem seems to be, what I would think is a cracked pickup tube. What has me perplexed is it doesn't matter if I hook it up to supply pipe on top of tank or return line right beside it, bubbles at just under 3/4. Initially I figured it must be cracked pickup since it ran fine all summer drawing thru the return line. Now it is doing the same thing drawing air at the same fuel level. So I am having a hard time thinking both pipes cracked at same height inside the tank or it be drawing a vacuum since I ran these tests with fuel cap off. I still think somehow it has to be a pressure issue, just don't at all see how. My next test will be to force air from the tap into the tank, is it possible the tubes are restricted? I do take extended time off, algae possibly? I can't see in the tank due to the anti siphon.

 

Yes this is my next move. More clear lines.

 

Just thought I'd update.

Clear line to fuel line direct to davco: bubbles slowly coming from the tank. Same if hooked to supply or return line. New vent valves installed both sides. All I can come up with from here is, inside the tank, are both pickup and return pegs welded together at that height, and they have corroded or otherwise made a pinhole in both pegs? My next idea is to replace the tank vent with a new pipe, if they're both same size and thread.

It is in the mechanic's hands now. Monday appt. God, my old mechanics closing shop really left a giant hole in the market for quality work done quickly. I would of paid them peterbilt price to keep them, bet a lot of others would of too.

 

In the pickup line AND the return line, at the same fuel level? That is my conundrum. When the problem first started I swapped it so it's pulling from the return and returning to the pickup. Worked fine til now.

Can't be a pressure issue, new vents and did all the tests with fuel caps off.

See what they come up with next week. It isn't in the driveway anymore.