Between Omaha and Des Moines, 7 miles South of I80 | 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.
 (M11%20Engine%20Diagram_fuel%20pump%20side.jpg)
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 M11%20Engine%20Diagram_fuel%20pump%20side.jpg (50KB - 615 downloads)
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Cummins M11 coolant line at front of exhaust manifold?
