[WarrenG]

Check all of your hose clamps also. I had the same problem last year. Burbing the system solved the problem, but at the same time I found one clamp just loose enough to seep without leaving anything on the garage floor. The only thing I could figure was that it let out enough coolant while driving to allow air into the system but wasn't a big enough leak to leave coolant on the floor when the engine wasn't running.