Correct me if I'm wrong, but you really shouldn't have all that much coolant mix in there in the first place. There is a max fill line that's about a 1/4 of the way up it. The purpose, being that it's an overflow tank, is if your car overheats, then the coolant has someplace to go. The increased temperatures create pressure on the radiator cap, which forces the spring in it up, allowing coolant to flow into the tank. If you've already filled it up, there would be no place for any overflow coolant to go.