Your Coolant reservoir bottles might be one of the first things you notice upon popping up the hood of your car due to the unusual transparent color it has. One of the most important functions of it is to maintain a constant pressure in your cooling system while you try to get to point B from point A. As the coolant heats up the rise in temperature will cause a rapid increase in pressure which is why most caps are purposely designed to control the pressure so the driver does not experience a sudden burst hose or valve. The reservoir bottle creates a vacuum that can draw coolant back in through the radiator cap making sure your cooling system always stays full of coolant and has no air bubbles.
