Why does brake fluid need changing if it never runs out?
Brake fluid does not get used up; it gets contaminated. Glycol-based fluids absorb water from the air, and every percent of water lowers the temperature at which the fluid boils. Boiling fluid creates vapour, and vapour compresses: the pedal goes long or to the floor.
Hygroscopicity
DOT 3, DOT 4 and DOT 5.1 fluids are glycol-ether based and hygroscopic: they absorb moisture through the reservoir vent, the seals and the flexible hoses. This is deliberate — water is spread through the fluid instead of collecting as droplets that could freeze or corrode a caliper — but it steadily lowers the boiling point. Fluid in a closed system still takes up water over time, which is why the interval is based on time as well as kilometres.
Dry and wet boiling points
The US standard FMVSS 116 sets minimum boiling points for new (dry) fluid and for fluid contaminated with about 3.7% water (wet), a level fluid can reach after a couple of years in service.
DOT 5 is not “better than DOT 4”
DOT 5 is silicone based. It does not absorb water and has a high boiling point, but it is not compatible with glycol fluids, it is more compressible, holds dissolved air, and is not used in modern ABS systems. DOT 5.1 is the glycol fluid with DOT 5 boiling points, and it can be mixed with DOT 4. Most modern cars specify DOT 4 or a low-viscosity DOT 4 variant for ABS and stability control.
Track and high-temperature fluids
Racing fluids are DOT 4 glycol fluids with much higher dry boiling points (well above 300 °C on some), but their wet boiling point is not dramatically higher. They are the right choice for track use, provided they are changed often: before an event, and after it if the car was used hard.
Practical rules
Follow the manufacturer’s interval, typically every two years, and change the fluid before any track day.
Flush the whole system, including the ABS unit where the procedure requires diagnostics, not just the reservoir.
A test strip or a boiling-point tester gives a real number; colour alone is not reliable.
| Fluid | Base | Dry boiling point (min.) | Wet boiling point (min.) |
|---|---|---|---|
| DOT 3 | Glycol | 205 °C | 140 °C |
| DOT 4 | Glycol | 230 °C | 155 °C |
| DOT 5.1 | Glycol | 260 °C | 180 °C |
| DOT 5 | Silicone | 260 °C | 180 °C |
Minimum values required by FMVSS 116. Many commercial fluids exceed them.