One of the most important facts about brake fluid is that it absorbs water - and this quietly drives why fluid must be changed on a schedule. Understanding why standard brake fluid does this explains a lot about brake maintenance. Here is the story.
Glycol fluid is hygroscopic
Standard DOT 3, DOT 4, and DOT 5.1 brake fluids are glycol-based, and glycol is hygroscopic - it chemically attracts and absorbs moisture from the air. Even in a sealed system, humidity slowly migrates in through microscopic pores in rubber hoses and seals and past the reservoir cap over months and years.
Why absorbing water is by design
It sounds like a flaw, but it is intentional. If the fluid did NOT absorb the moisture, that water would pool as free droplets in low spots of the system, where it could freeze or, worse, boil into a pocket of vapor and corrode metal locally. By absorbing water throughout, the fluid keeps it dispersed and manageable - a lesser evil.
The downside: lower boiling point
The trade-off is that absorbed water lowers the fluid's boiling point over time - see brake fluid boiling point. Old, wet fluid can boil under hard braking and cause a soft pedal and fade - see signs fluid is boiling. Moisture also promotes internal corrosion of expensive parts.
Why this means scheduled flushes
Because moisture creeps in on a calendar, not by mileage, manufacturers set a time-based flush interval - see how often to change brake fluid and signs of contaminated fluid. Fresh fluid restores a high boiling point. Silicone DOT 5 does not absorb water, which is why it is used differently. Keep fluid fresh alongside quality pads and hardware.