Originally Posted by selim View Post
Your overflow tank level would be more important if you had a closed system (no radiator cap). But since you don't have a closed system, you need to always check both. I think that you probably let the coolant level in the radiator drop to low and it was unable to create a vacuum to siphon coolant from the overflow tank. Since you let the system drop so low, you really should purge any air from the system. If you're not sure how, just check the levels for the next couple of days and top off as necessary. This should take care of it.
So keep the radiator and overflow at the correct levels, you should have no problems. And if you're using water...I strongly recommend you do a complete flush and refill with the proper coolant mix.