Brake fluid is replaced on a time interval rather than a mileage one, which is unusual among service items. The reason is that it degrades whether the car is driven or not.
How a hydraulic brake system works
Pressing the pedal pushes fluid through sealed lines to the calipers, where it forces pistons against the pads. The system depends on the fluid being effectively incompressible.
Liquids resist compression well, so pedal movement translates almost directly into clamping force at the wheels.
Gas does not. Any vapour in the system compresses under pressure, so pedal travel is consumed squeezing bubbles instead of moving pistons.
Why water gets in
Most brake fluids are hygroscopic, meaning they actively attract and absorb water from the surrounding air.
Moisture enters slowly through flexible hoses, past seals and through the reservoir, and the fluid draws it in and holds it in suspension.
This is deliberate. Water dispersed through the fluid is far less dangerous than water collecting as droplets at the lowest point of a line, where it would freeze or corrode.
What absorbed water does to boiling point
Fresh fluid boils at a high temperature. Absorbed water lowers that figure substantially, and the drop continues as more moisture accumulates.
Braking converts vehicle movement into heat at the disc, and some of that heat reaches the caliper and the fluid inside it.
On a long descent or after repeated hard stops, fluid that has absorbed moisture can reach its lowered boiling point, producing vapour and a pedal that travels much further for much less braking.
Why the interval is measured in time
Absorption depends on exposure to humid air rather than on distance covered, so a car used lightly ages its fluid at a similar rate to one used constantly.
A vehicle standing for long periods in a damp environment can end up with worse fluid than one driven daily.
That is why the schedule is expressed in years, and why low mileage is not a reason to postpone the service.
What else the fluid protects
Brake fluid carries corrosion inhibitors that protect the metal surfaces inside master cylinders, calipers and the components of the anti-lock system.
Those additives deplete over time, after which internal corrosion begins on parts that are expensive to replace and cannot be inspected without dismantling.
Replacing fluid therefore preserves the hardware as well as the performance, and a system left long enough will need more than fluid to put right.