Replacing brake fluid: why and when

What does brake fluid actually do?

Your brake system is hydraulic: when you press the brake pedal, the fluid pushes through the brake lines and turns that pedal force into force at the calipers, which press the pads against the discs. So brake fluid is literally what transfers your pedal force to the brakes — without good fluid, your car doesn't brake.

Fluid barely compresses, and that's the whole point: all the force from your foot goes straight to the wheels. But if there's air or moisture in the system, part of that force is lost to compression instead of braking — and you feel that in the pedal.

Why does brake fluid need replacing?

Brake fluid is hygroscopic: it slowly draws moisture from the air, through hoses, seals and the reservoir. Over time there's measurable moisture in the fluid — and that's exactly the problem. Water has a much lower boiling point than brake fluid.

Brake hard or long (downhill, with a trailer, in traffic) and the brakes get hot. If the fluid holds a lot of moisture, that moisture can start to boil and form vapour bubbles. Vapour does compress — and at that moment your brake pedal suddenly feels soft or spongy and the car brakes less. So over time the safety margin of your brake system drops, even if the fluid still looks fine. Replacing it restores the boiling point.

When should you replace brake fluid?

Rule of thumb: replace brake fluid about every two years, or per the interval your manufacturer prescribes — that can vary by make and model. It's about time rather than mileage, because the moisture comes in gradually, even when the car is standing still.

The best measure is a moisture test: we measure the moisture content in the fluid. If it's above the safe limit, replacement is needed — even if the interval hasn't passed yet. We check the brake fluid as standard at the APK (Dutch MOT) and service, so you don't have to keep track of it yourself.

Warning signs

Brake fluid ages quietly, so you often only notice once your brake system already has less margin. Watch for these signs and have your brakes checked:

• Soft or spongy brake pedal — the pedal feels soft or springy instead of firm.
• Longer pedal travel — you have to press the pedal further before you feel any effect.
• Brake warning light — a light on your dashboard pointing to the brake system.
• Less sharp braking — the brakes bite less directly, especially under hard or repeated braking.
• Low fluid level — a dropping level in the reservoir can also point to wear or a leak.

How AutoJet helps: test, replace, check

Not sure whether your brake fluid is due for replacement? At AutoJet in Almere we test the moisture content for free. If it's above the safe limit, we replace the fluid to your car's spec and bleed the system, so no air is left in it and your pedal is firm again.

Since we're at the brakes anyway, we check your pads and discs at the same time — so you know where you stand in one go. Enter your license plate and we see the make and model and give you a clear price up front. We speak Ukrainian, Russian and English — handy if Dutch isn't your first language.

The exact price depends on your car — enter your license plate and you'll see it up front. You never pay for work you haven't approved in advance.

See also: Brake service in Almere · Blog

Frequently asked questions about replacing brake fluid