Its air tight enough. Alcohol base fluid, will take on moisture.
Most master cylinder boots are designed to drop with the fluid,vent in cap is to allow air in above the boot, designed to work like a bellows.
I agree, but still it's not air tight as far as moisture. That's the whole reason for recommended brake fluid changes, because it gets contaminated over time. It won't hurt a thing to leave the cap off a day or two. He can put a shop cloth over it if he's skiddish.
