Brake fluid is one of those things that can escape us. When a car needs a service there are things we think of straight away; oil, oil filter, air filter, spark plugs. But how often do you think about your brake fluid? Exactly. And we at Haynes don’t want you to come a cropper. As such, we’ve put this guide together for you, so you know when and how to change your brake fluid.

How often should I change my brake fluid?

Well, this is a tricky one to answer, as it varies from car to car. The general rule is that brake fluid should be good for four to five years. However, if you have a heavier car you'll need to knock a year or two off that, while a lighter car might get a bit more life out of its fluid. The best and most definitive way to know is to check your trusty Haynes Manual.

One thing to consider about brake fluid is that even if you’re not changing it, you should regularly check it. As a bare minimum, you should be giving it a cursory inspection once a year. Which leads us neatly to…

How can I tell if my brake fluid needs to be changed?

A common misconception is that brake fluid will last forever. That’s simply not the case. Like engine oil, it can degrade over time. It does this by absorbing moisture, which decreases its effectiveness in the process. Not only that, moisture will corrode the braking system from the inside out, and that’s the last thing you want.

Furthermore, if the car is used particularly hard, the fluid can get too hot and in extreme cases, even boil. This will affect the chemical structure of the fluid, and in the process will make it less effective. In a nutshell, it loses its viscosity and becomes too thin to work properly. Brake fluid is carefully engineered to have a certain thickness, hence the 'DOT' rating.

Your brake fluid should always be clear. If it has any grain or murkiness, it needs to go. If it smells burned, it also needs to go. Other indicators will be if the level drops on the reservoir (it reduces as the brake pads wear), or if you notice you have to press the pedal harder to get the car to stop. 

How (and when) to change your brake fluid

How to change your brake fluid

Obviously, each car will be ever so slightly different, or subject to its own intricacies. However, the general procedure remains the same, which we’ll go through now. Before we do, there is one caveat: get a second person to help you. Yes, there are ‘one-man’ bleed kits, but they’re seldom as effective as having a mate pressing on the pedal for you. 

Also, you’ll need to know if your car has ABS or not. If it does, you’ll need to bleed the system as below, but with the engine running, because the ABS pump will need to be flushed and primed, too. Check your car’s Haynes Manual for more information. 

1) Drain the old fluid

Take the cap off the brake master cylinder and, using a syringe or something similar, remove as much of the fluid as possible. Put this old fluid in a suitable container. Once you’ve removed as much as you can, it’s time to go to the brakes themselves. The best way to tackle this is with the car up on axle stands, with all four wheels removed. 

2) Fill it back up

Now you need to get a lint-free cloth and clean out any debris that might be in the reservoir. Once that’s done, you need to fill the master cylinder back up with new fluid, right up to the MAX line. Make sure you use the correct-specification fluid. Don’t put the cap back on though, because this can create a vacuum, which isn’t what you want when bleeding. You’re looking to push the old fluid out, not suck it back in. 

3) Flush the calipers

Each brake calliper has a bleed nipple on the back. You’ll need to release this to let the old fluid out, but before going at it with the spanners, spray it with some brake clean then penetrating oil to free it up. It’ll be full of road dirt. Affix a small length of rubber hose to the nipple, and then submerge the end of the hose into the container you put the old brake fluid from the master cylinder in. 

4) Time for a mate

Stick a small block under the brake pedal (this stops your mate from pushing it to the floor, which is too far). With your helper in the driver’s seat, go to the prescribed caliper (usually the one FURTHEST from the master cylinder to start with, but check your Haynes Guide to be sure) and open the bleed nipple. Shout “DOWN” and get your helper to repeat when he or she has pushed the pedal down. Now close the nipple, shout “UP” and have him or her do the same when they’re off the pedal. If they come off the pedal while the bleed nipple is open, you’re just going to suck air into the system – the very thing we’re trying to avoid. 

With each down pump, you’ll see the fluid escape from the nipple. You will also see any air bubbles, and that’s good, as we need to get rid of them. Keep doing this until the new fluid is coming through, without any bubbles. 

5) Top ups

Keep an eye on the fluid level in the master cylinder, because you will need to top this up as you progress. Do not pump the pedal if it gets too low, because again, you’ll just suck in air. 

6) Rise and repeat

Do the same on each caliper, working closer to the master cylinder as you go. By this stage, you should have a decent pedal ‘feel’, by which we mean the pedal should feel firm. This is good, because it means the brake system is full of decent quality fluid. All you need to do now is check the level in the master cylinder one last time, top off if needed, then close it up, put the wheels back on and be on your way.