Brake fluid changes explained: when and why they matter

Brake fluid is the hydraulic fluid that carries your foot pressure from the pedal to the brakes at each wheel. It is easy to forget because it sits in a small reservoir and never seems to run out. But it does not last forever, and old fluid is one of the quieter causes of poor braking.

Most brake fluids attract water from the air through the hoses and seals. Over time that moisture lowers the temperature at which the fluid boils, and it encourages corrosion inside the pipes and calipers. Here is what a fluid change involves and how to judge when one is due.

Why does brake fluid need changing?

Most brake fluids are hygroscopic, meaning they draw in moisture gradually. Water in the fluid can turn to vapour when the brakes get hot, for instance on long downhill stretches or after repeated heavy stops. Vapour compresses, so the pedal feels soft or goes low just when you need the brakes most.

Moisture also attacks metal parts from the inside. Calipers, wheel cylinders, metal pipes and the ABS unit can corrode, which can result in seized parts or leaks. Changing the fluid on time helps avoid that damage, though the exact cost of the work depends on the vehicle.

How often should brake fluid be changed?

It depends on the vehicle. Many manufacturers specify a time interval, often measured in years, rather than a mileage, because the fluid ages whether or not the car is used. The service schedule in your handbook gives the correct interval, and a garage can also test the fluid.

If you do not know when it was last changed, and the car has no service history to show it, it is reasonable to have the fluid tested. Cars used for demanding driving, such as hilly routes or heavy loads, may benefit from checking it more often.

How can I tell if my brake fluid is old?

Fresh fluid is usually clear to pale amber. Old fluid darkens and may look brown or even black. You can look at the reservoir with the engine off, but colour alone does not show how much water the fluid has absorbed, so a test is more reliable.

A garage can use a tester that measures moisture content or boiling point. Avoid opening the reservoir more than necessary, because exposing the fluid to air lets it take on moisture faster.

• Dark or cloudy fluid in the reservoir
• A pedal that feels softer after heavy braking
• No record of a change within the manufacturer interval
• Corrosion marks around bleed nipples or fittings

What happens during a brake fluid change?

The garage uses the correct fluid specification for your car, which is shown on the reservoir cap or in the handbook. Old fluid is drawn out of the system and replaced wheel by wheel, normally in a set order, until clean fluid comes through. This also removes air, and the pedal is checked afterwards.

Cars with ABS need care so that air does not enter the control unit. While the wheels are being worked on, the technician can look at the pads, discs, hoses and calipers. Cost drivers are the make and model, the system layout, the amount of fluid and any seized bleed nipples. At Automaster, a garage at Longcombe Cross near Totnes, we can include a fluid test in a brake check.

Is it safe to top up or mix brake fluid?

Topping up a slightly low reservoir with the correct fluid is acceptable in an emergency, but a falling level suggests worn pads or a leak and should be investigated. Mixing different specifications can reduce performance, and some types are not compatible with each other.

If the pedal feels spongy, sinks, or a red warning light comes on, do not keep driving. That points to a more serious fault than old fluid, and the car needs to be inspected before further use.