Read back to what I told you. My bets are combustion is getting into the system causing the air. You can get a simple test kit to see if it is combustion gas.
Test kit 
I agree, you may possibly have a head gasket issue.
A compression test and a block leak test would help identify if that's the problem.
Any idea if the water pump has ever been replaced?
Make sure that the weep hole on the bottom of the water pump isn't seeping coolant.
You said that steam was coming from below the engine.
The wife's Charger was using some coolant slowly and it was the water pump weeping coolant very slowly.
