I read over your original post again,
I did not notice that you said you changed out all the cooling system components earlier.

What exactly were you chasing as a problem then?

You can try rinsing out the the port holes of the cap with a water hose. That should be plenty of pressure to open it up and flush out any scum around the components insde it.

I think a "little" shot of air may not be bad either. Just refrain from trying to launch it! (:-)

You have ruled out a lot of other options except to test the system.
I think you might be able to rent the combustion tester.. Don't ask me where?
Or buy one anywhere for around $40 or less nowadays.

The only other thing I can think of, is that maybe when you changed out the cooling system, you somehow trapped a whole lot of air in there.
You might only see a result of vapor trails, not a great quantity unless you are above the maximum line. I like keeping the level in between the two lines.

How long has it been since the work was done?
The air should burp out quickily as it will rise to the highest point of the system after every shutdown.

Keep us posted of what you find!

Phil