The coolant is heated and expands as the vehicle is driven. The expanding coolant flows into the coolant reservoir. The valve in the pressure cap opens due to expansion and coolant flow.

Any air or vapor in the cooling system will be forced into the coolant reservoir through a hose. The air and vapor will leave through the vent at the top of the reservoir. As the system cools, the extra coolant in the reservoir will be drawn back to the cooling system through the valve in the radiator cap. In this manner, the radiator will remain full at all times. Check the level of coolant in the reservoir to see if more coolant needs to be added.