Bottle, Pressurized Coolant: Operation
WARNING: This page is about a different car, the 2017 RAM 2500 HD. However, it is still accessible from the selected car via links, so may be relevant.
The pressure bottle works in conjunction with the cooling system pressure cap to keep the coolant free of trapped air. It provides a volume for expansion and contraction, a convenient and safe method for checking coolant level, and adjusting coolant level at atmospheric pressure. It also provides some reserve coolant to cover minor leaks and evaporation or boiling losses.