Brake fluid: when to change it, and why almost everyone forgets
Updated · Autofy
Brake fluid: when to change it, and why almost everyone forgets
Most carmakers ask for brake fluid to be replaced about every two years, whatever the mileage. The reason: it absorbs moisture from the air over time, which lowers its boiling point. On a long descent or under repeated braking, water-laden fluid can form vapour bubbles and the pedal goes soft. Topping up isn't enough: the system has to be bled and all the fluid replaced, to the standard in your service book (usually DOT 4).
A fluid that ages without driving
Brake fluid carries the force of your pedal to the calipers. It's hygroscopic: it gradually absorbs moisture from the air, through the hoses and the reservoir cap. That happens whether the car is driven or not, hence an interval in years rather than kilometres.
Why water is a problem
Braking turns the car's energy into heat, and some of it reaches the fluid. As long as it doesn't boil, all is well. But water lowers the boiling point: for DOT 4 fluid, the standard requires at least 230 °C when new, versus 155 °C once it has absorbed moisture.
When the fluid boils, it forms vapour bubbles that compress: the pedal goes soft and sinks. It happens at the worst moment, on a long mountain descent or during repeated hard braking. Water also promotes corrosion in the calipers and ABS unit.
The interval: about two years
Most carmakers call for replacement every two years, some every three. It's the most forgotten service, because no light or noise reminds you. The only figure that counts is the one in your service book.
Signs of tired fluid
- A softer pedal or one that travels further, especially when hot.
- Dark, brown fluid in the reservoir: fresh fluid is clear and amber.
- An unknown last replacement date, or one more than two years ago.
Note: a low level doesn't mean old fluid, it means worn pads or a leak. Our brake warning light article explains the difference.
Topping up isn't replacing
Adding fluid to the reservoir changes nothing about what's already in the system. Replacement is done by bleeding: the old fluid is pushed out of the calipers until fresh fluid appears, wheel by wheel. It's quick, and often done during a service.
Use the standard your carmaker specifies, usually DOT 4. DOT 5, silicone-based, isn't compatible with the others.
Never forget it again
This is exactly the kind of job that slips your mind. Autofy keeps the date of every service with a photo of the invoice, so you never have to wonder "when was it last done?". For wear, see also when to replace brake pads.