Brake fluid: why it needs changing every two years.

Pads and discs get the attention because you can see them wear and hear them complain. Brake fluid is out of sight, and it is the part of the braking system that deteriorates simply with time, whether the car covers 3,000 miles a year or 20,000. That is why most manufacturers specify a change at set time intervals, with two years being a common figure.

What brake fluid does

When you press the pedal, you are not pushing the brakes directly. You are pushing a piston in the master cylinder, which pressurises fluid in the brake lines. That fluid pushes pistons in the calipers (or wheel cylinders on drum brakes), which clamp the pads onto the discs.

For that to work, the fluid has to transmit force without squashing. Liquids are almost incompressible; gas and vapour are not. That is the root of most brake fluid problems.

Why it absorbs water

Most brake fluid in cars is glycol-based and graded DOT 3, DOT 4 or DOT 5.1. These fluids are hygroscopic, which means they draw in moisture from the air. A small amount gets in through the reservoir, and a small amount through the rubber hoses, which are slightly permeable. Even a perfectly healthy system takes on water slowly over the years.

Fresh fluid has a high boiling point. As it absorbs water, the boiling point falls, and the more water, the bigger the drop. Specifications for each DOT grade give a minimum "dry" and "wet" boiling point, and the wet figure, which represents fluid that has absorbed some water, is considerably lower. That is the reason for an age-based interval.

Why boiling matters

Braking turns movement into heat, and much of that heat ends up in the brakes. Under heavy or repeated braking, such as a long downhill, towing, or driving a heavy car hard, the calipers get very hot, and that heat passes into the fluid.

If the fluid has absorbed enough water, it can boil in the caliper. Boiling creates vapour bubbles, and vapour compresses. The result is a soft, long or sinking pedal, and reduced braking when you need it most. When the brakes cool down, the pedal may feel normal again, which can mislead you into thinking nothing was wrong.

This is the reason the issue is a safety one rather than a comfort one. On a normal drive you may never notice. On the one occasion you need to brake hard repeatedly, you could.

Corrosion and component wear

Water in the fluid also encourages corrosion inside the system from the inside out. It can affect:

• Calipers and caliper pistons
• Brake pipes and unions
• Master cylinder bores
• ABS modulators and valves, which are expensive components with fine tolerances

Old, contaminated fluid can also darken and carry debris. Replacing it at the right interval is generally cheaper than replacing a seized caliper or an ABS unit.

How often should it be changed?

Two years is a widely used interval, and many manufacturers specify it, but not all do. Some use a mileage interval or a different time period. Your handbook or service schedule is the best guide for your car. If you do not know when it was last changed, and the car is more than a couple of years old, it is reasonable to assume it is due.

Heavy use, such as towing, track days, mountain driving or spirited driving, can justify more frequent changes.

Which fluid should be used?

Always use the grade the manufacturer specifies, which is usually printed on the reservoir cap or in the handbook. Common points to know:

• DOT 3, DOT 4 and DOT 5.1 are all glycol-based, but use the grade your car specifies rather than mixing them.
• DOT 5 is silicone-based and must not be mixed with the glycol types. It is rare on ordinary road cars.
• A higher grade is not automatically better, and some systems are designed around a specific fluid, so stick to the specification.
• Brake fluid is damaging to paintwork and should be handled with care. Always dispose of old fluid properly.

Can I just top it up?

Topping up is not the same as changing it. The fluid level in the reservoir falls slowly as the brake pads wear, which is normal. A level that falls quickly or needs frequent topping up can indicate a leak and should be inspected promptly. Adding fresh fluid to old fluid does little to restore the lost boiling point.

What does a brake fluid change involve?

The old fluid is removed and replaced with fresh, correct-grade fluid, and the system is bled so that no air is left in the lines. The exact procedure depends on the car; some with ABS or electronic parking brakes have specific steps. A fluid test, using a tester that measures water content, can also show whether a change is due.

Signs it may be overdue

• A soft or spongy pedal, or one that sinks slowly under pressure
• A pedal that feels worse after hard braking
• A brake warning light
• Fluid in the reservoir that is dark or dirty rather than light amber
• No record of it ever having been changed

None of these means fluid is the only cause, so have a proper inspection if you are unsure. Our guide to brake warning signs covers other symptoms and what they usually mean.

In short

Brake fluid is cheap compared with the parts it protects. Change it on schedule, use the right grade and mention it if you notice a change in pedal feel.

Talk to us

A brake fluid change is £59. See our brakes page for details, or call us on 07746 922 273. You may also find our guide to brake warning signs useful.