It depends. Brake fluid absorbs moisture, which lowers its boiling point. You don't want it boiling from brake heat when you're really trying to stop. If you live somewhere humid then the whole reservoir can slowly absorb too much moisture and make the fluid unsafe.
Correct. Brake fluid absorbs moisture but the key is that DOT 3, 4, and 5.1 fluid then spreads that moisture evenly throughout itself, so that you never have a pool of water in your brakes but instead a dispersed low percentage of moisture in the fluid.
So even if water only enters the system at the caliper or reservoir, that moisture still spreads through all the fluid in the entire car. That's why you flush it all out when changing brake fluid.
No need to panic. An entire generation never refreshed this fluid and vapour lock was still a rare incident. Replacing 20% of this fluid every year is more than enough.
