Brake Fluid: The Small Autumn Job That Matters on Long Descents

The end of the driving season is a good moment to look at the one fluid most owners forget. Brake fluid does not wear out with mileage in the way oil does. It ages with time, and a car that has spent the summer on passes and track days has asked more of it than the odometer suggests.

Why brake fluid ages

The glycol-based fluids used in almost every road car are hygroscopic, which means they absorb moisture from the air. Water finds its way in through the reservoir cap, the seals and the flexible hoses. It does not need a leak, only time.

Water lowers the temperature at which the fluid boils. If the fluid in a caliper boils, it forms vapour, and vapour compresses where liquid does not. The pedal goes long and soft, usually at the bottom of a hill or the end of a straight, which is the worst place to find out.

What the DOT numbers mean

The DOT grades come from the United States standard FMVSS 116, which sets two minimum boiling points for each grade: dry (new fluid) and wet (fluid that has taken on water in a set test).

• DOT 3: 205°C dry, 140°C wet
• DOT 4: 230°C dry, 155°C wet
• DOT 5: 260°C dry, 180°C wet

The gap between the two columns is the point. A DOT 4 fluid can lose 75°C of margin through moisture alone. DOT 5 is silicone-based and must not be mixed with glycol fluid. DOT 5.1 is the glycol fluid made to the same boiling figures. Use the grade named in your handbook or on the reservoir cap.

Why a Lotus still needs it

A light car is kind to its brakes, and an Elise will go down an alpine pass with far less drama than a heavy saloon. The fluid is still what carries the heat. The long descent from the Stelvio or a twenty-minute session at a track day will find old fluid in any car, and the bigger brakes of the Exige V6 do not change that.

When to change it

Follow the interval in the service schedule for your car, and treat it as a time limit, not a mileage one. If the car is used on circuit, change the fluid more often. Many track day drivers do so every season.

Autumn suits the job. The car goes into winter with fresh fluid, and it is ready for the first dry day of spring.

• Buy fluid in a sealed container and do not keep an opened bottle for next year.
• Brake fluid damages paint. Wipe any spill at once and rinse with water.
• A racing fluid still absorbs water, so it needs changing just as regularly.

For the other checks worth making before a trip, see the road trip preparation guide and the brake section of the track day modifications guide.

Source: FMVSS 116, Motor vehicle brake fluids (49 CFR 571.116).