Brake fluid is replaced on a time schedule rather than a distance one, which puzzles owners of low-mileage cars. The reason is that the fluid degrades whether the car is driven or not.
The fluid is hygroscopic by design
Most brake fluids are glycol based and actively attract water from the atmosphere. Moisture enters through flexible hoses, past seals and through the reservoir.
This sounds like a flaw and is in fact deliberate. Water that entered the system would otherwise pool at low points and freeze or corrode the components there.
By dissolving moisture and holding it in suspension throughout the fluid, the system avoids concentrated pockets of water, at the cost of gradually degrading as a whole.
Absorbed water lowers the boiling point
Braking converts kinetic energy into heat at the disc, and some of that heat conducts into the caliper and the fluid immediately behind the piston.
Fresh fluid boils at a high temperature. A few per cent of absorbed water reduces that figure substantially, and the reduction begins from the moment the system is sealed.
The specification quotes both a dry boiling point for new fluid and a wet one representing fluid that has absorbed a defined amount of moisture over its service life.
Boiling produces a soft pedal, not weak brakes
Hydraulic systems work because liquid does not compress. Vapour does, so a bubble of boiled fluid absorbs pedal travel instead of transmitting force.
The driver feels the pedal go long or spongy, and pumping it may partially restore pressure temporarily as vapour is displaced.
The failure appears precisely under heavy or repeated braking, which is when the system is most needed, and it disappears once the fluid cools, leaving no trace.
Corrosion runs in the background
Water in the system attacks metal components from the inside, particularly in calipers, wheel cylinders and the bores of the master cylinder.
Anti-corrosion additives in the fluid slow this and are depleted over time, so an old fluid offers less protection even before its boiling point is considered.
Antilock and stability control units contain fine passages and precision valves, and corrosion products in those assemblies lead to expensive failures.
Why testing is imperfect
Electronic testers measure conductivity or a related property and infer moisture content, which gives a useful indication but not a direct boiling point measurement.
Moisture is not evenly distributed either, since fluid at the calipers has been exposed to more heat and often holds more water than the fluid in the reservoir.
That uneven distribution is why a full flush through each caliper is specified rather than simply drawing fluid out of the reservoir and topping it up.