Not to be argumentative, but I've always questioned the notion that air bubbles going into the overflow tank creates air in the system. Afterall where did those bubbles come from in the first place, it is a closed system? So the air may be moving around, and seen as tiny bubbles in the tank, but it was already there. In other words, the coolant going into the tank did not create a new source of air added to the system. If anything, the air going into the tank may allow it an opportunity to collect there and be more easily bled by venting the cap.