M1008 Getting air under load

[Payback]

Hello everyone, new to the group but have been lurking for the passed few months.
I recently picked up a 86 m1008 cucv and have been doing some work on it to make it a bit better, new brakes, new fuel tank, restored and painted the underside, and a new coat of paint on the body. It's been going fine, until now..
Long and short of it is that the 6.2 was running pretty good after all the work so I pulled it in the shop to paint the body. Ran like a champ, in the shop for a few weeks for paint, pull it out when i was finished and took it for a ride. Got to 55 mph and it hit the wall, started loosing power, stumbling, stalling. After looking around I find the procedure to bleed the fuel lines and looking for leaks in the system. Bleed the lines, use a clear tube at the IP to confirm no air, take it out to the road and at around 35 mph it falls on it's face again. Pop the hood to look at the clear tube, AIR? Let it idle a bit to clear the air out, roll down the road and air comes back?
So, at idle and revving the motor in park, no air, strong sound, put it in gear and go for a ride, put a load on the motor and at the same rpm all the time air is introduced. Like at the same level of needing more fuel, air starts to enter? Then let it idle for a while ( 1 min.) air disappears power comes back to almost 100 %
I am pretty sure it is not from lines going to and from the fuel tank,no leaks under pressure. Is this a IP problem that I could be having? I can use any advice or direction in getting this solved, I can use any help I can get.
( I did take to a very respectable diesel shop to be repaired, they had it for three weeks and never pulled it into their shop to look at it. Had to go get it and bring it home so I could try to fix it myself.) . Thanks for all your time. Thanks!

 

[cucvrus]

When was the injection pump last rebuilt. I just put 1 on yesterday. It is $315. that was well spent. It is fairly easy and a bit time consuming. How about some pictures.

 

[nyoffroad]

I too recommend an IP But before that check all the lines from the tank to the fuel pump on the engine block, those lines are NOT under pressure and may allow some air in. If it's sucking air in it most likely would also leak fuel when shut off but who knows? Did you REPLACE the fuel lines and clamps? Also possible but unlikely you didn't bend and crack the fuel pick up when changing the tank did you? Like I said unlikely but I did see that once.

Another thought! Check the oil level and small it for fuel, if the diagram in the mechanical pump is leaking it will put fuel into the engine oil and I think it may also suck air in. Just a guess.