Brake fluid: why it ages, when to change it, and what it costs

What brake fluid does

When you press the brake pedal, your foot doesn't touch the brakes. A liquid does the work:

1. 1The brake pedal pushes a piston in the master cylinder, helped by the brake servo.
2. 2Brake fluid fills the master cylinder, the brake lines and each wheel's caliper. Liquids can't be squeezed, so the pressure travels instantly to all four wheels.
3. 3At each wheel, the fluid pushes the caliper's piston, which presses the brake pads against the disc.

The fluid sits in a small reservoir on top of the master cylinder, usually at the back of the engine bay on the driver's side, with a yellow or black cap marked with the fluid type (such as "DOT 4"). On many manual cars, the clutch uses the same fluid and reservoir.

Why brake fluid goes off

Most brake fluids (DOT 3, DOT 4 and DOT 5.1) are hygroscopic: they absorb water. A little moisture gets in all the time, through the brake hoses, the seals and the air in the reservoir, whether you drive or not. That's why brake fluid is changed by age, not distance.

The water does two kinds of harm:

• It lowers the boiling point. New DOT 4 fluid must boil no lower than 230 °C. With about 3.7% water, which brake fluid standards call "wet", the minimum drops to 155 °C. Brakes turn movement into heat, and the fluid in the calipers gets very hot during repeated hard braking. If it boils, it forms vapour bubbles. Unlike liquid, vapour can be squeezed, so the pedal sinks and the brakes don't bite.
• It corrodes the brakes from inside. Water attacks the caliper pistons, the master cylinder and the ABS unit, parts that are far more expensive than a fluid change.

When to change it

Every 2 years, whatever your mileage, is what motoring clubs such as the ADAC recommend, and what most manufacturers specify. Some new cars have the first change after 3 years, then every 2 years. Your service book has your car's interval.

Change it every year if you drive hard (mountains, towing, track days) or live in a very humid or coastal climate.

How many kilometres you drive and how you drive day to day don't change the interval: brake fluid ages with time. Your maintenance level does.

Which brake fluid does your car need?

The type is printed on the reservoir cap and in your owner's manual. The DOT classes set minimum boiling points:

• Most European cars use DOT 4. Some use DOT 5.1, often labelled "DOT 4 / 5.1".
• Use exactly what your manufacturer specifies. DOT 3, 4 and 5.1 are all glycol-based, but don't swap or mix them unless your manual allows it.
• Never put DOT 5 (silicone) in a car that uses DOT 3, 4 or 5.1, or the other way round. They don't mix, and the wrong fluid can damage the seals.
• Only use fluid from a sealed bottle. An opened bottle absorbs water from the air.

How to check your brake fluid

You can check the level in a minute, but not the fluid's age or water content.

The level

• Look at the reservoir with the car on level ground. The fluid should be between the MIN and MAX marks. Most reservoirs are translucent, so you don't need to open them.
• A slightly lower level is normal as the brake pads wear, because the caliper pistons move out and hold more fluid. Don't top it up to MAX without a reason: when new pads are fitted, the level comes back up.
• A level near or below MIN, or one that keeps falling, means very worn pads or a leak. Have the brakes checked straight away.

The condition

• Colour is only a rough clue. New fluid is clear and pale yellow; old fluid turns amber, then brown. Dark fluid is due, but clear-looking fluid can still contain water.
• Cheap test pens dipped in the reservoir give a rough idea of the water content, but they aren't reliable enough to skip a change.
• A workshop can measure the boiling point with a proper tester. That's the only reliable check, and it's quicker and cheaper to just change the fluid on time.
• Don't leave the cap off. The fluid absorbs moisture from the air, and it damages paint: wipe up any drops straight away.

Warning signs

Old brake fluid rarely gives any warning before it fails, so don't wait for signs. If you notice any of these, have the brakes checked:

• The brake warning light (an exclamation mark in a circle) stays on with the parking brake released: often a low fluid level.
• A soft or spongy pedal that goes further down than usual.
• The pedal sinks slowly while you hold it down at a standstill.
• The brakes fade after repeated hard braking, for example on a long downhill road.
• Fluid leaking near a wheel or under the engine bay: a clear to brown oily liquid.
• Dark brown fluid in the reservoir.

What happens if you don't change it

1. 1Nothing you can feel. The fluid absorbs water quietly, year after year, and the brakes feel the same in everyday driving.
2. 2A lower boiling point. The brakes still work normally, but the safety margin for hot brakes shrinks.
3. 3Corrosion inside the brakes. Calipers, the master cylinder and the ABS unit start to corrode. Sticking calipers and a failed ABS unit cost far more than a fluid change.
4. 4Brake fade on a long descent. After repeated hard braking, the fluid boils, vapour bubbles form and the pedal goes soft.
5. 5The pedal goes to the floor. In the worst case, there's so much vapour that the brakes barely work, just when you need them.

Why not just wait for a sign?

• There usually isn't one. Water in the fluid is invisible, and the brakes feel normal until the fluid boils.
• Low mileage doesn't help. Moisture gets in whether the car is driven or not.
• A full reservoir isn't fresh fluid. The level tells you nothing about the water content.

How brake fluid is changed

1. 1The old fluid is drawn out of the reservoir and the reservoir is refilled with new fluid of the right type.
2. 2The car is raised and the wheels are removed or turned for access.
3. 3Each brake is bled, usually starting with the wheel farthest from the master cylinder: a small valve on the caliper is opened, and new fluid is pushed through until it comes out clean and without bubbles. Workshops use a pressure bleeder; at home it takes a helper pressing the pedal.
4. 4The reservoir is kept topped up throughout, so no air gets in.
5. 5On some cars, the ABS unit is bled with a diagnostic tool, and the clutch is bled too if it shares the fluid.
6. 6The level is set and the pedal checked: it should be firm and high before the car moves.

At a workshop it takes about 30 minutes to an hour. Old brake fluid is hazardous waste: it must never go down the drain.

How much a brake fluid change costs

A litre of brake fluid costs about €15, the same everywhere, and the job takes about 45 minutes, so the price depends mostly on the workshop's labour rate. Figures are estimates for an independent workshop, VAT included.

Questions people ask