Radiator Cap
The reservoir has a pressure cap, which is designed to maintain pressure in the cooling system. As the engine warms up, coolant temperature increases and the coolant expands. The pressure cap prevents the expanding coolant from escaping and results in a pressure increase in the system. This raises the coolant's boiling point. This allows the cooling system to maintain the correct engine operating temperature without boiling over.
The radiator cap has both pressure and vacuum valves. At the rated pressure, the pressure valve opens, to allow vapor to escape into the atmosphere and coolant to flow into the reservoir due to expansion. This prevents excess pressure from building up in the system.
When the engine is stopped and allowed to cool off, the pressure in the cooling system drops from a pressure that is higher than the atmospheric pressure, to one that is lower. At this point, the vacuum valve opens to allow ambient air to enter the reservoir, and pressure in the system equals atmospheric pressure. If the coolant level in the radiator is low during this period, coolant will be drawn into the radiator.