Besides the obvious, I am having trouble understanding the difference between the pressured cooling system and the non pressured system, and how they differ in function. I have the reservoir that looks like the pressured system. Why is it embossed with the word gas? Why does it appear to have immediate flow of coolant when the vehicle is started? I have low compression on number 2 cylinder and by all indications it appears to be a valve problem, but I need to eliminate the idea that it could be a breech in the lower intake gasket. With coolant flowing immediately after starting is this compression gas or is it normal operation? I know there is a hydrocarbon test I could do, but it would simplify my thought process knowing how the system functions.