How Brake Fluid Works & How To Top It Up

If there’s one part of your car you need to work as well as possible all the time, it’s probably the brakes. Without them you’ve got a bit of a problem.

There are various parts of your brakes that work together, one of the most important being the brake fluid. If your brake fluid is low or has deteriorated in quality then it won’t work as well, which could have safety implications when driving.

So let’s delve a little deeper into what brake fluid does and how you can check it and top it up…

What is brake fluid & how does it work

Brake fluid is used in hydraulic brakes and works by transferring the force applied when pressing the brake pedal to the wheel.

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When you press the pedal, brake fluid is forced into a cylinder which pushes a piston. This piston then moves the calipers to push the brake pads onto the brake disc, causing friction which slows the wheel down. Without brake fluid, there’s nothing to push the piston and work the brake pads. This happens simultaneously on all four wheels when you press the brake pedal.

Brake fluid is also used for general lubrication to reduce wear.

How does brake fluid get low?

There are two main reasons your brake fluid levels might be low. The main reason you may have low fluid levels is that you have a leak somewhere. This is a very serious problem, so if you suspect a leak you need to get it checked out as soon as possible.

If you get a low brake fluid warning in your car, it could also be because your brake pads have become worn. When the pads wear out, the piston needs a little extra help to push them onto the brake disc, and so requires more brake fluid to be taken into the cylinder.

If you feel this is the case then you should top up with a small amount of fluid and then check the brake pads, getting them replaced if necessary. Don’t fully top up, however, if your brake pads are worn – when you fit new pads there will then be too much fluid, causing it to overflow. You can top up fully once you have new pads.

Brake fluid reservoir

How does brake fluid deteriorate in quality?

When you apply the brakes and the fluid is forced around the hydraulics, it heats up. If it heats up too much, it can vaporise. However, it needs to remain as a liquid in order to push the pistons.

Brake fluid is hygroscopic, which means it absorbs water from the atmosphere. This water lowers the boiling point of the fluid, making it more likely to vaporise and affect the effectiveness of the breaks.

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It’s recommended that you replace your brake fluid every two years at the most, although if you have an annual service, they should do this for you.

There are various types of brake fluid, containing different mixtures of chemicals that affect the boiling point.

What are the different types of brake fluid?

There are four different types of brake fluid – DOT 3, DOT 4, DOT 5 and DOT 5.1 (DOT stands for Department of Transport).

Each of these contains different chemicals which dictate the boiling point:

  Dry boiling point     

     Wet boiling point   

  DOT 3 

205 °C (401 °F)

140 °C (284 °F)


230 °C (446 °F)

155 °C (311 °F)


260 °C (500 °F)

180 °C (356 °F)

DOT 5.1 260 °C (500 °F)

180 °C (356 °F)

The dry boiling point is when using pure brake fluid. The wet boiling point is the temperature when there is 3.7% water in the mixture, showing how important it is to keep it ‘clean’.

DOT 3, DOT 4 and DOT 5.1 are polyethylene glycol-based, whereas DOT 5 is silicone-based. Please check your owner’s manual as to which performance level your car requires as some types are not suitable for all cars – for example, DOT 3 should not be used in mineral oil brake and suspension systems fitted to some Citroen, Jaguar and Rolls Royce models.

You can see just a couple of the types of brake fluid we have available below.

Carlube DOT 3 brake fluid

Buy Carlube DOT 3 brake fluid

Carlube DOT 4 brake fluid

Buy Carlube DOT 4 brake fluid

Some people may tell you that DOT 3 and DOT 4 for are interchangeable, but this isn’t strictly true. You may be able to use DOT 4 instead of DOT 3 but never the other way round.

Check out our full range of brake fluid and cleaner.

How to check & top up your brake fluid

The first thing you need to do is locate what’s known as the brake master cylinder reservoir. Do this when the car is cold so you don’t burn yourself on anything under the bonnet.

Unfortunately there is no specific place the master cylinder and reservoir will be, although most of the time it will be at the back of the engine bay on the driver’s side.

On newer cars, the reservoir will be semi-transparent, allowing you to see the fluid levels, and there will probably be ‘min’ and ‘max’ levels. Obviously, if it’s looking a little low then simply top up with fluid and you’re good to go.