[HUF]

Under normal conditions the expansion tank (1) will take care of increased volume of hot coolant. After the radiator cools down the vacuum inside it will suck up the coolant from the expantion tank back to the radiator. Nevertheless, if for some reason coolant gets the boiling point (say, a failed water pump) the overflow tank (1A) should take care of boiling coolant and keep it from being deposited on the ground.That said the overflow tank (1A) should be normally empty and the expansion tank (1) half empty.