[undy]

The water in my expansion tank is always right up to the very top of the neck. I also have an overflow tank that the expansion tank is plumbed into. As that car heats up and the coolant expands it allows the fluid to overflow into the overflow tank. As the car cools it allows the engine/expansion tank to "pull" the coolant from the overflow tank back into the expansion tank. This effectively makes it a "closed" system. Among other benefits the main benefit is that you're not drawing air back into your cooling system which helps to curtail corrosion in your cooling system.