This thing has been bugging me for weeks. I've been burping my cooling system every few days (at first, I probably wasn't doing it right, but now I think I've got it), and it seems to take care of the problem. The next day, I see the bubble in the down-tube of my overflow bottle again! It's smaller now than it was, but it's still there. I'm about to pull my hair out! The only leak I can find is a few drops of coolant on the underside of the overflow tube where it is attached to the radiator cap neck. Could this leak be letting air into the down-tube? Should I add some hose clamps? Anyone know of anything else that could be keeping me from getting rid of the bubble? Should I keep doing what I'm doing until it finally goes away? Thanks guys!