Brake fluid has one job spec that matters more than any other: its boiling point. Because brakes turn motion into heat, the fluid can get hot enough to boil - and boiling fluid means a scary loss of braking. Understanding dry vs wet boiling points explains why fluid is a maintenance item.
Why boiling point matters
Brake fluid works because liquid does not compress. If it boils, the vapor bubbles that form DO compress, so your pedal goes soft and stopping power drops - a form of brake fade. This is most likely during sustained hard use when brakes run hot - see how hot brakes get and what is brake fade. A higher boiling point is a bigger safety margin.
Dry vs wet boiling point
Every fluid lists two numbers. The dry boiling point is fresh, moisture-free fluid; the wet boiling point is after the fluid has absorbed about 3.7% water over years of service. The wet number is always much lower - and it is the one that matters, because your fluid spends most of its life partly wet. DOT 3 wet is around 284F, DOT 4 wet around 311F - see DOT 3 vs DOT 4.
Why it drops over time
Standard brake fluid is hygroscopic - it pulls moisture from the air through hoses and seals. As water content climbs, the boiling point falls steadily, which is exactly why manufacturers set a time-based flush interval regardless of mileage - see how often to change brake fluid. Old fluid can boil far sooner than fresh.
The takeaway
For most drivers, keeping fluid fresh with a periodic flush maintains a safe boiling point cheaply - see the flush guide. If you tow or run mountains, a quality DOT 4 gives more headroom. Fresh fluid plus good pads and hardware is the complete heat-management package.