Very strange problem you have...
Have you replaced your radiator cap with a stant 10230 or 11230 as recommended by GM (don't use the acdelco/gm)
Its very puzzling that air can build up in the system but not displace antifreeze. When you say the levels are right on are you checking the expansion tank, the radiator, or both? perhaps you are leaking antifreeze at the same rate air is coming in?
Regardless, its a pressurized system so if any air could get in then it would have to be above 15lbs so I feel the exhaust gas check already recommended is valid and wise in your case.
Please let us know the results.