Although air is not "in the system" normally, there is air between the cap and top of the fluid in the master cylinder. There is also a vent in the master cylinder cap. The cap is there only to keep fluid from spilling out, it's not an air tight system. That's not how it works. I do agree brake fluid does absorb moisture. But it's designed to. At least everything but DOT5. Were it not for fluid sloshin all over the place, you could throw the masted cylinder lid in the ditch and go.
