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 brake fluid works
Unless you drive something really, really old, your car is going to have hydraulic brakes. And that’s good, because hydraulic braking systems are commonplace and also the most effective. However, they’re only any good if they’re maintained properly.
Yes, there is the matter of discs and pads, but don’t go forgetting the fluid. Without this, your brakes won’t do anything. When you press the pedal, you act upon the fluid, which then in turn acts upon the brakes.
The problem with brake fluid is that it’s hygroscopic. That means it likes to suck up moisture. The more moisture that becomes a part of the brake fluid, the lower the boiling point of the fluid becomes. So, logic would dictate that the longer you leave it, the less effective it will become.
Can brake fluid go bad in any other way?
If there is a fault with your braking system at any point, and the fluid gets too hot, you can boil it. The lower the fluid boiling point, the easier it will boil. If the fluid boils, lots of air bubbles form, which will allow the fluid to be easily compressed. If the fluid can be compressed, it can no longer act upon the brakes when the pedal is depressed. The pedal becomes ‘soft’ and the brakes no longer work. This can happen after a period of prolonged braking, such as descending a long, steep hill. In fact this can also happen when the car is driven enthusiastically (on a race track, for example), where the brake fluid can overheat and cause something called brake fade, where the hydraulic action of the fluid is less effective.
Take the cap of the brake master cylinder and smell the fluid, if it smells burned, or if it’s exceptionally dark in colour, it’s time to change it.
Your Haynes manual will guide you through the process of how to change the brakes, and it will also cover what brake fluid your car needs. While some fluids are technically interchangeable, it’s always best to make sure you’re filling your braking system with the right stuff.
How often should I change my brake fluid?
Well, this is a tricky one to answer, because it varies from car to car. Most vehicle manufacturers recommend the brake fluid is changed every 2 years, regardless of the mileage. It may be that the fluid would be good for longer, but why take the chance?.
Even if the fluid looks okay, you should still change it. You’re not going to see the moisture in the fluid – it happens on a microscopic level. 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 every few weeks. Which leads us neatly to…
How can I tell if my brake fluid needs to be replaced?
Brake fluid is carefully engineered to have a certain thickness, hence the 'DOT' rating you'll see on the bottle when you buy it.
Your brake fluid should always be clear. If it has any dirt 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 goes down as the brake pads wear, so don't automatically think that there's a leak), or if you notice that the brake pedal is spongier than it used to be when pressed.
How to change your brake fluid
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. There are ‘one-man’ bleed kits, but they’re seldom as effective as having a friend pressing on the pedal for you.
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. Only use new fluid from a sealed container. If the container is open, the fluid will absorb moisture from the air. What brake fluid do you need for your car? Consult your Haynes manual or your car's manual to make sure you use the correct-specification fluid.
3) Flush the calipers/wheel cylinders
Each brake calliper/wheel cylinder 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 cleaner 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) Get help
Stick a small block under the brake pedal (this stops your friend from pushing it to the floor, which is too far). With your helper in the driver’s seat, go to the prescribed caliper/wheel cylinder (usually the one farthest 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, because we need to get rid of them. Keep doing this until the new fluid is coming through, without any bubbles.
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/wheel cylinder, 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.
Dispose of the old brake fluid sensibly. Don't pour it down a drain or tip it onto waste ground. Take it to your local council's recycling centre; you'll be directed to the right place when you drive in.