Inside Your Car 9 July 2025

Everything to know about brake fluid

Brake fluid is what helps transfer the pressure you apply to the brakes, all the way down to the wheels. You may not think about brake fluid in your day-to-day life, but it is an important part of your car’s braking system.

If these fluids haven’t been changed for a while, it can drastically reduce the effectiveness of your brakes—which can be dangerous. Let’s take a dive into all things brake fluid so that you can be sure of a smooth and safe drive.

What is brake fluid?

Brake fluid is a type of hydraulic fluid within your car’s braking system. It’s a non-compressible fluid, and without it, you wouldn’t have braking consistency when hitting the brake pedal, so it’s pretty important.

When you press down the brake, it pushes on a piston inside the brake master cylinder. The brake fluid is then forced down through the brake lines to your wheels, where the brakes are applied.

The pressure created by your brake fluid is therefore crucial to the proper function of your brakes. It also needs to withstand a lot of heat caused by friction, and over time it wears out and needs replacing.

What colour is brake fluid?

The most common types of brake fluid—DOT 3, DOT 4, and DOT 5.1—are all a clear light amber when fresh. DOT 5, which is mainly used in military vehicles and motorcycles, is a very light purple colour.

All of these will darken with age as they take on moisture, which is one indicator that they need flushing out and new fluid added.

How to check brake fluid

It’s important to check your fluid once a year to ensure that it’s in top shape. As brake fluid is hygroscopic, it takes on water from the atmosphere over time, which decreases its boiling point. This is bad news for your brake system as boiling introduces air into the system, leading to spongy (and sometimes squeaky) brakes. The fluid can also become contaminated with debris.

Checking your brake fluid is fairly straightforward:

When checking the fluid, you need to look under the hood on the driver’s side. The brake fluid reservoir sits right on top of the brake master cylinder and it usually has a bright yellow cap.
Wearing gloves, clean the cap and the surrounding area so no grit falls into the fluid, then remove the cap. This should have a good seal, so check this at the same time.
The fluid should be up to the ‘max’ line. If it’s low, it could be an indication you have a leak.

The brake fluid should be light golden or light purple. Both of these fluids degrade to a dark brown over time. Once it gets dark it needs to be changed.

If you want a more exact measurement, you can use a boiling point tester. Insert this into the reservoir, connect it to the battery, and the tester will boil a small sample of the fluid and display its boiling point. There are also pen testers that are a little more simple to use, but aren’t always as accurate.

How to flush brake fluid

Syphon all the brake fluid out of the reservoir with a vacuum pump. Be careful as it’s corrosive and will damage your paint (and skin!) if spilled. Alternatively, your local mycar mechanic can do this safely for you.

1. Fill the reservoir up to the top with new fluid, loosely replacing the reservoir cap.

2. Safely jack up the car and take the wheels off.

3. There’s a bleeder screw on the top of the calliper which you’ll need to loosen and attach the vacuum pump to.

4. Be sure to create a tight seal so you don’t let any air into the system.

5. Pump out the old fluid until you see fresh fluid come down the lines from the reservoir.

6. Keep topping up the reservoir from time to time as well, so you don’t let any air in that way.

7. When the fluid coming into the vacuum pump is light golden, you can close up the bleeder screw and continue with the other wheels.

Here’s how to replace brake fluid

Flush out the old fluid (read above)
Fill the reservoir on the brake master cylinder right up to the ‘max’ line
Replace the cap, ensuring that no grit is around the seal, and that’s it!

How do I know what brake fluid I need for my car?

The type of fluid you need will be printed on the cap of the master cylinder reservoir, but if you’re unsure you can always drop into your local mycar.

Different brake fluids boil at different points. If you’re going hard on the brakes or towing heavy loads, chances are you’ll need a higher number fluid.

Brake fluids fall into two camps - glycol-based and silicone-based, which should never be mixed due to their different properties. Most cars use glycol-based fluids, with silicone-based brake fluid being used in military vehicles and in show cars.

The most common types of brake fluid you’ll find are:

DOT 3, DOT 4, and DOT 5.1: These are all glycol-based, with a higher number indicating a higher boiling point (and price point!).
DOT 5: This is silicone-based. This won’t work in ABS braking systems, as it becomes foamy and makes your brakes spongy.

Need some help changing your brake fluid? Call one of our dedicated mechanics and book in for a service today!