How to check brake fluid, and when it needs changing

Why it matters

Brake fluid is what actually carries the force from your foot to all four wheels. The master cylinder turns the pedal force into hydraulic pressure, and the brake lines carry that pressure to each wheel, so the fluid has to stay free of air bubbles and excess moisture.

Brake fluid absorbs moisture from the air over time, even in a sealed system, and that lowers its boiling point. Under hard braking it can boil into vapor, and vapor compresses, so the pedal can feel soft at the worst possible moment.

Signs it needs attention

• The level sits at or near the min marking, or is lower than it was
• The brake warning light is on, which can mean low brake fluid
• A soft, spongy or unusually low brake pedal
• Fluid that is due by age, however clean it looks
• On a manual car, a clutch pedal that feels spongy or soft

How to check it, step by step

1. Park on level ground with the engine off.
2. Find the brake fluid reservoir and read the level against its min and max markings.
3. If the level is low, note it and have the reason found. A falling level can mean the pads have worn or that there is a leak.
4. If fluid is added, use only the type in your owner's manual. Different types are not always compatible, and the wrong one can damage seals in the system.
5. Look for visibly cracked, bulging or leaking flexible brake lines, where you can see them.
6. Check when the fluid was last changed, and book a change if it is past the interval in your manual.

What the reservoir is telling you

• The level is between min and max: fine for now, but the fluid is still changed on its time interval.
• The level is lower than it was a few months ago: this can be the pads wearing, which is normal, but have a leak ruled out.
• The brake warning light is on: check the parking brake is fully released. Otherwise it is possible low fluid or a hydraulic issue, so stop if the pedal feels wrong.
• The level looks fine, but the pedal is soft or spongy: possible air or moisture in the fluid. Have it checked immediately.
• The fluid looks clean, but it is past its interval: still due, because moisture contamination is not visible.
• Electric or hybrid car: the fluid still ages on a clock. Regeneration does not slow moisture absorption.

What people get wrong

That a low level just needs topping up. Topping off low brake fluid without investigating why it is low skips the question that matters: whether the pads have worn, which is normal, or the system is leaking, which is not.

When to leave it to a workshop

Checking the level is reasonable to do yourself. Bleeding or flushing the fluid is not: air in the lines is a serious safety issue, and even experienced home mechanics generally leave brake fluid bleeding to a shop with the right tools. The same goes for anything else involving the brake hydraulic system, and for diagnosing a soft or spongy pedal.

Read next

• Soft or spongy brake pedal: why it is urgent
• Brake warning light: check this first
• Brakes squealing or grinding: what the noise means

This page is general information and does not replace a professional assessment. Symbols, codes and behaviour vary between makes. Always check the handbook for your own car.