I apologize to those of you who already know this, but I just learned today that the pressurized plastic coolant tank on my 2012 Mazda3 serves primarily to keep air out of the cooling system. It is mounted higher than the rest of the system. It has a relatively low pressure cap that allows air out when it exceeds a certain pressure and lets air in when the vacuum exceeds a certain pressure. I have posted a photo below. The two small barbs are inlets. One is an engine return line connected to the thermostat housing and the other is a radiator return line. The lower, larger barb returns coolant to the radiator. It is internally baffled to "burp" air from the system. I was ignorant before and now feel educated.