Brake Fluid Change: Your Comprehensive Guide

A mechanic carrying out a brake fluid change on a car's engine

You might book your MOT on time every year. You might even follow a regular car service schedule. But if you haven’t booked a brake fluid change in the last couple of years, you’re neglecting the most important part of your vehicle.

Yep, brake fluid is often a forgotten aspect of vehicle maintenance – but you put your safety at risk by doing so.

That’s why we’re here. We’ve put together an easy to follow but thoroughly comprehensive guide about everything to do with a brake fluid change. We’ll cover what it is, why it’s important, how it works in relation to the rest of your braking system and, most importantly, how you can keep your brakes in top working order.

Let’s get straight to it.

How Does Brake Fluid Influence How Your Brakes Work?

This diagram shows just how complicated the braking system actually is.


A diagram of your car's brakes


Confusing, isn’t it? Let’s break it down and explain the basics.

For starters, each wheel has its own individual brake. All four need to work in unison with each other to stop your car effectively. The front brakes are far more important than the rear brakes because they bear the brunt of the car’s weight under braking. Usually, this means you have far more efficient disc brakes at the front and drum brakes at the rear.

When you press the brake pedal, a piston in the master cylinder is depressed. Most modern cars have four of these cylinders fitted, two for each hydraulic circuit, just in case one should fail somewhere in the system. The dual system allows for greater force transfer into the front brakes and shorter braking distances.

This system cannot work without brake fluid. No fluid = no pressure.

And pressure is what makes the whole thing work. Without pressure, you won’t be stopping any time soon.

The fluid forces pistons out of the slave cylinders onto the brakes. It’s this arrangement that allows so much force to transfer from the slave cylinder to the brake because the pushing area is much greater. It also means each slave cylinder only has to travel a fraction of an inch, allowing for faster braking.

Click here to find out more about how your brakes work.

The Composition of Brake Fluid

Brake fluid has an incredibly high boiling point to deal with the friction and heat generated under braking. Time, wear and tear and unnatural degradation can all affect the boiling point – which can lead to significant problems.

At this point, it’s worth making something clear – not every braking system needs brake fluid.

We described how a hydraulic braking system works but not every car uses one of them. An electromagnetic braking system, for example, doesn’t require a brake fluid change. There is no brake fluid used as the process is completely frictionless. If you’re not sure what braking system you use, check under your bonnet. If you have no brake fluid reservoir, your car has electromagnetic brakes fitted.


brake fluid reservoir


‘Lost’ Brakes Affect Safety

The composition of brake fluid leaves it susceptible to turning ‘bad’.

It absorbs water and moisture as it travels through the system, which lowers its boiling point. And that lowers its efficiency.

Brake fluid works best as a liquid and absorbing too much water lowers the boiling point. That’s when things get dangerous as you can ‘lose’ your brakes when you’re trying to slow down. The car won’t slow down when your brakes are ‘lost’ as the pedal will just flop straight to the floor.

What’s more, rust and other gunk can build in your brake lines reducing the efficiency of your brakes and potentially causing a leak.

Remember, no fluid = no pressure which means your car won’t stop when you want it to. And there’s no way you can drive your car without brakes.

Make Sure You Choose the Right Type!

As with everything, there’s a range of brake fluids available and each one is different. Therefore, it’s important to remember that you can’t mix and match them!

Each car is designed to work with a specific type of brake fluid, so bodging a brake fluid change can leave you with as many problems as an incorrect oil change.

The only exception to this rule is if you need to add brake fluid in a hurry, such as to get to a garage in the event of a leak. When you get to the garage, it’s important to let the mechanic know what you’ve done and then ask them to flush the system completely. Once your other problem is fixed, make sure the correct brake fluid is used. You can find which one you need in your vehicle’s handbook.


Different types of brake fluid bottle.


DOT 3, 4 and 5.1 can all be used in regular vehicles and, if absolutely necessary, compatible with each other. DOT 5 is off-limits to all cars except those designed to run it. It doesn’t attract water, thus preventing rust, but is incredibly expensive and doesn’t offer you many benefits. The main reason it can’t be used with any of the other types of brake fluid is that it’s a silicon-based fluid and all the others are glycol-ether based. So, it’s basically a chalk and cheese situation which will only end up costing you money.

If you do need to top your brake fluid up yourself, only buy a small bottle of the correct type. Why? Well, air doesn’t mix with brake fluid. At all. And contaminated brake fluid is no use to anyone. In an unopened container, brake fluid can last up to two years, but it’s contaminated almost instantly when you open it. If you’ve got an old bottle lying around, it’s best to dispose of it rather than use it for a brake fluid change. Your brakes might not work if you use old fluid.

Regularly Book a Brake Fluid Change for Best Results

The best time frame for a brake fluid change is once every two years. Leaving it any longer between flushes can be dangerous because of the potential water content in your brake fluid lines. As an example, DOT 4 brake fluid usually has a boiling point of 230 degrees Celsius – when it’s wet, that drops to 155 degrees Celsius.

It’s a lot easier to ‘lose’ your brakes at that temperature.

Braking generates a huge amount of friction, so this difference is both significant and dangerous. Don’t neglect a brake fluid change! What’s more, if you ever ‘lose’ your brakes, you need to book an appointment with a mechanic immediately. Just don’t drive yourself to the garage.

A leak in the brake fluid lines could be to blame. A leak that will cause your brakes to stop working altogether.

Call your recovery provider and ask them to tow you instead.

Where Can You Book a Brake Fluid Change?

Fortunately, you don’t need to look very far. Here at BookMyGarage, many of our garages offer a brake fluid change as a fixed price product – so you’ll always pay roughly £30 – £70 depending on the garage you choose. When you choose one of our garages, you choose a high-quality service delivered at the best possible price. Why not search our directory for your next brake fluid replacement today? You can even book online in just a few clicks while you’re at it!


So, there you have it, everything you need to know about a brake fluid change. Have you learnt anything new? Maybe you’ve got some more advice for drivers still sceptical about the importance of their brake fluid? Let us know in the comments below, and don’t forget to book your next appointment today!

Mandy Weston

Mandy is an ex-mechanic, with 22 years’ experience in the motor industry. As an in-house motoring expert, Mandy is the go-to woman for any relevant questions that our customers have; both garages and drivers. From specific problems with your car to general maintenance, Mandy is a reliable source of information and advice. Her passion for motoring is a huge factor to her success and the huge wealth of knowledge that she has. She now uses her remarkable grasp of the industry to write regular content for our readers to help drivers understand their car better, avoid being ripped off by garages and save money on their motoring requirements.

4 Replies to “Brake Fluid Change: Your Comprehensive Guide”

  1. Levi Armstrong says: Reply

    Thanks for informing me that the cause of losing breaks is when too much water lowers the boiling point, that’s why it’s important to change brake fluids once every two years. As a new car owner, I did not know that this was necessary to maintain my car. But now that I know this, I’ll make sure to take my car to an auto service that does brake repairs to change the brake fluids since I haven’t had it changed for about three years now.

    1. Mandy Weston says: Reply

      Hi Levi,

      Thanks for the kind words! Glad we could help you learn something new, be sure to check back in for more top tips! 🙂

  2. How do I differentiate between needing a break fluid flush and needing a new or repaired brake master cylinder?

    1. Mandy Weston says: Reply

      Hi Evelyn,

      Brake fluid should be changed every 2 years as it absorbs water which will increase the boiling point of the fluid and potentially damage your hydraulic braking system.

      The brake master cylinder provides the pressure for a hydraulic braking system. Low brake fluid, a brake warning light, spongy brake pedal or the brake pedal drifting slowly to the floor are all indicative of a brake master cylinder fault. This is because the seals have failed and are causing the cylinder to lose pressure. You may also see contamination in the brake fluid from the seals or air bubbles from the air ingress in the cylinder.

      As with any brake query, we strongly recommend booking a professional brake check as soon as possible if you have a fault.

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