Sealing surface for the cap OK?
Where the gaskets sets?
Flanges and locking notches and tabs deformed at all?
Radiator shop can probably repair if needed.
$1k...

I have had them repaired at a radiator shop as well. Does it only do it when on plane? Never does it sitting still?

The top flange on tank is little rough and was a concern that not getting good seal but it did seal good when I used my pressure gauge. Think it would help to double up rubber seals on the cap?

I can't see the HE being the problem if it holds 20 psi. My HE had one tube leaking into the raw water side. Testing at 15psi it was VERY noticeable which tube was leaking along with pressure dropping.

When you originally psi tested your closed cooling system, you connected the correct cap onto your expansion tank and pressureized to 15psi? No leakage or drop in psi? This tells me the problem is not in the closed cooling system.

No I pressurized system via removing cap and pressurizing the tank. I never thought of rigging up a way to pressurize system with my cap on and seeing if I can get the cap to leak. There is a small hose that runs from tank to top of heat exchanger wondering if I cap one side then pressurize through hose would work?

Correct. By doing this you tested from cap down and had no leaks/psi drop. Correct again, install expansion tank cap. Pressurize system from this hose and dead end HE fitting. Another option is to install a Tee in this line and pressurize. This way will test entire system including suspect cap and ecpansion tank.

Congrats