There is a small, about 3/8" coolant line at the front of the exhaust manifold. Not connected to the exhaust, but just in front of it. It is attached to some piping that goes to the over flow tank on the truck's fire wall. Its a 95' FLD 112 is that makes a difference.

 What is the purpose of this coolant line? Is this an aeration elimination line, or?....... It gets warm before any other coolant lines.

 Thanks.

Your describing a coolant vent line going to a coolant expansion tank.

Bleeds off some small amounts of air, if there is some in the cooling system. Also, helps to bleed air, during the initial fill of coolant anytime a system was refilled.

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 Perfect. Thanks for that. That is just what I needed.

 I have some air getting into the system. I bought a combustion gas tester and it is not combustion getting into the coolant, but I think it is the air comprssor. The air then over pressurizes my expansion tank.

 Thanks again.

Can you isolate the air compressor water/coolant lines(cap those lines), and just let the air compressor free-wheel (leave off the air compressor's discharge line, so it can't pump any air), THEN see if you still have bubbles in expansion tank, with engine idling?

Also, Some of those older trucks, used an "air style heater control valve" and they can leak "truck system air pressure" into the cooling system at heater hose areas.

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 Hmmm,.... interesting. I did not know about the heater idea. I will look into that. I did pull a plug on the air tanks so the compressor could not build air, but I did not loop the coolant lines. The compressor does get warm rather quick and it is inconsistent at how it builds pressure.

 Thanks for the tips.