When to change brake fluid

Safety first: if the brake pedal feels soft or spongy, sinks further than usual, or the brake warning light comes on, stop driving as soon as it is safe and have the brakes checked. Do not wait for the next service. See our brake warning light guide.

Why brake fluid needs changing

When you press the pedal, brake fluid carries that force through the brake lines to the brakes at each wheel. It has to cope with a lot of heat: Swansway Garages notes it must withstand temperatures of up to 230°C (about 450°F).

The catch is that most brake fluid is hygroscopic: it slowly absorbs moisture from the air. That water does two kinds of damage:

• It lowers the boiling point. Under long or repeated hard braking, wet fluid can boil and form vapor bubbles. Vapor squashes where fluid does not, so the pedal goes soft and the car takes longer to stop.
• It causes corrosion inside the brake lines, calipers and the ABS unit, which are much more expensive to replace than the fluid.

This happens gradually, whether you drive a lot or not, which is why many carmakers set a time limit rather than a mileage.

How often to change it

There is no single rule, so your owner's manual or service book is the place to check. As a guide:

• Every two years is the most common advice. UK garages commonly change it when the car is two years old and every two years after that, regardless of mileage. Volkswagen and Mercedes-Benz have long specified two years on most models.
• Some makers use mileage, for example Subaru at 30,000 miles50,000 km.
• Some set long intervals or none. Chevrolet has given up to 10 years on many models, and some Ford, Chrysler and Toyota schedules do not list a routine change at all. In that case a garage can test the fluid at each service and tell you when it is due.

If you tow, drive in the mountains or brake hard a lot, ask for the fluid to be tested more often.

Signs your brake fluid needs attention

• A soft or spongy brake pedal, or one that sinks too far before the brakes bite. Stop and get it checked.
• The brake or ABS warning light on the dashboard. On many cars this can mean the fluid level is low.
• The level is below the MIN mark on the reservoir under the hood. A low level can mean there is a leak somewhere in the system.
• Dark, cloudy or dirty-looking fluid. New fluid is clear or pale yellow.
• The car takes longer to stop than it used to, or pulls to one side when braking.

What it costs

DIY or garage?

How to keep it simple

1. Look up the brake fluid interval in your owner's manual or service book.
2. Check the reservoir level about once a month, with the car on level ground.
3. Ask the garage to test the fluid at each service.
4. Log the date of each change in our service tracker.