Motorcycle Brake Fluid Guide

There’s no denying that brake fluid is an extremely important part of a motorcycle’s brake system make-up.

The fluid is what transfers the force we apply on the lever into pressure, so making sure that we have the correct fluid and that it is well maintained is something that every rider should be conscious of.

Brake fluid has its own performance life and there are also different types available to us.

This guide will detail why and how often we should change it, as well as look into some of the different brake fluid types available for us to use.

Why brake fluid needs to be changed

Glycol-based brake fluid (the most common for cars and bikes) is hygroscopic. This means that the fluid will aggressively absorb moisture.

As soon as the seal is broken on the bottle the brake fluid will begin to absorb moisture from the air. Even once it’s sealed in your system (though it’s never truly sealed) it will continue to absorb moisture from things like rubber seals and hoses.

Over time as more and more moisture is absorbed you will get a build-up of water in the system, and it is that water that changes the performance characteristics of the brake fluid.

Fluid in its natural state is not appreciably compressible, but when that fluid is vaporised by high temperatures the resulting vapour can be compressed. This means the fluid is then less able to convert force into pressure.

It is for this reason that brake fluids have very high boiling points to combat this, but when moisture is absorbed into the fluid the boiling point drops dramatically.

DOT 4 brake fluid for example has a boiling point of around 230 degrees centigrade, but once the fluid absorbs enough moisture to the point where it is then made up of just a few percent water, the boiling point can drop as much as 80 degrees C.

This is why it needs to be changed.

How often to change it

For road use, it is recommended that your brake fluid be change every 1-2 years. This would more than likely be completed as part of your service schedule.

On the track however you do not want to have any chance of experiencing any kind of performance drops given the level at which you rely on the brakes, so changing it earlier wouldn’t be a terrible idea. Around once a year if not more if you do quite a lot of track days.

People will comment it should be changed a lot more often because it experiences a lot more heat, but when buying the best fluids from the best brands you shouldn’t experience any performance drops due to overheating.

The different types (DOT fluids)

The brake fluids of today must meet set requirements in order to be fit for sale. These requirements come from organisations such as SAE International, or the local government equivalent.

The USA for example generally follow the SAE standards, but make amendments to compensate for the massive differences in environments between states e.g. Florida vs Alaska. It is the US Department of Transportation that classifies the fluid, which is where the term DOT comes from.

Being in the UK, the fluid sold here is also sold as DOT fluid.

There are a few different types such as DOT 2 (castor oil-based), DOT 3, 4, 5.1 (Glycol-based) and DOT 5 brake fluid (silicone-based – not a recommended racing brake fluid).

For motorcycle use though – particularly on the track – there are only two types you need to concern yourself with, and it will more than likely be one of the following fluids that is recommended for your motorcycle.

DOT 4 vs DOT 5.1

DOT 5.1 is more hygroscopic than DOT 4 so it absorbs more moisture. Because of this it needs to be changed more often.

A benefit of 5.1 is that it has a higher boiling point, meaning it can experience higher temperatures than DOT 4 before it starts to experience any adverse effects. Having said that, a good quality DOT 4 fluid should be able to contend with the temperature levels it would experience on a track day.

DOT 5.1 will also maintain a lower viscosity in lower temperatures, right down to around -40 degrees C. Though again, this should have no standing on UK track days.

Which is better?

5.1 will perform better at the extremes of use, but at the level at which your motorcycle is going to perform on a track day then I believe that a good quality DOT 4 brake fluid is going to perform to just the same level.

It’s like the semi vs fully synthetic oil debate, a fully is going to give you slight benefits in performance, but a semi synthetic is more than good enough for the job.

Neither is ‘better’ and both make fine performance brake fluids, but one option simply makes more sense for track riders.

Can you mix them?

Rather than flushing the system empty and refilling (meaning you’d need to bleed the brakes), there’s no reason why you can’t use your new type of brake fluid to fill the reservoir as you pump out the old stuff – when going from DOT 4 to DOT 5.1 or vice versa, that is.

You should however make sure you get rid of all the old fluid. Putting 250ml+ into the system should be enough to see all the old stuff out.

When you come to slam on the anchors after hammering your bike down the back straight you want to be damn sure your brakes are going to stop you. Having your brake fluid well maintained and in good condition is a very important thing to have in check to allow that to happen.

There are obviously other links in the braking system chain, but keeping on top of your motorcycle brake fluid maintenance is one very important link to have doing its part.