How To Replace Motorcycle Brake Fluid and Why
Your owners manual calls for it, likely every two years, but why do we need to do this and how can we do it ourselves? I will get into the why a bit, but I won’t leave you hanging on the “how’ if you want to get straight to it. The video below shows you the steps you can take to do it on your own. It doesn’t take many tools, and with what dealerships are charging these days, you might even be able to buy the tools and the fluids for less than they would charge you.
Why Change Brake Fluid?
Your braking system is the most critical aspect of your bike to maintain. It is your number one tool in avoiding accidents and keeping you in one piece. Shortening your braking distances as little as a foot can literally mean the difference between life and death.
What role Does Fluid Play In my Braking System?
Brake systems work on principles of hydraulics, converting pressurized oil—specifically brake fluid in this case—into mechanical energy. The brake lever produces a significant amount of…well, leverage onto your master cylinder, which in turn forces brake fluid through the lines and into the calipers. From there, the fluid pushes on the brake piston(s) forcing the brake pads against the rotors. The friction of the pads against the rotors then converts the mechanical energy of the bike’s inertia into thermal energy or in layman’s terms, heat. And it’s that last bit of info that makes changing brake fluid on regular intervals so critical.
All that heat transfers from your pads directly into the calipers and eventually into the brake fluid itself. The fluid I use in the video is Motul RBF600, the RBF standing for Racing Brake Fluid. What makes it suitable for the rigors of racing is its very high “dry” boiling point of 594 degrees Fahrenheit. What is the “dry” boiling point? Isn’t brake fluid inherently wet? Well, there’s wet, and then there’s wet. What is meant in this case by the word dry is a freshly opened container of brake fluid. Like many things (everything?) it doesn’t stay fresh forever. The fluid absorbs water vapor from the air, making it “hygroscopic” in nature. If you hear someone say “hydroscopic” you have my permission to slap them, its not a word. What comes after that is on you, but I digress. Water, as you hopefully know, boils at just 212 degrees and for our purposes that is a very bad thing.
In order to perform well, our hydraulic system relies on fluid that can not be compressed. If it can be, all that force exerted by the lever on master cylinder simply goes to compressing vapor, not pushing fluid. And if the fluid doesn’t move, the brake pads don’t either, and if you’re already in motion, so will you—probably much further than you wanted to go and possibly into something very solid. No matter what, water will find its way into the braking system and as it does, the performance of the fluid is compromised. That brings us to the “wet” boiling point. The Department of Transportation defines wet brake fluid as being comprised of the seemingly random amount of 3.7"% water. For the fluid we chose, the boiling point for wet fluid drops down to just 399 degrees, a nearly 200 degree difference. Relative to most brake fluids, that is actually a very high wet boiling point. It will continue to decrease unless we take action and swap the old fluid for fresh.
Which Fluid is Right for My Bike?
First and foremost, check your brake fluid reservoir cap. It will tell you which DOT number fluid to use. For both of our bikes, and most bikes in general, it will call for DOT 4. This is critical, because not all fluids perform the same or even get along with one another.
Check your brake fluid reservoir cap for the minimum DOT brake fluid standard.
DOT 3 fluid uses glycol ether as a basis and was used on earlier cars and bikes produced up until the late 1990s. It meets the specifications for those vehicles. You don’t have to settle for DOT 3 however just because that’s what was fine in 1982.
DOT 4 fluid adds borate ester to the mix. It became standard for all vehicles around 2006. Its composition is similar to that of DOT 3 and they are even compatible so they can be mixed. This is good news if you have an older bike and want to upgrade, but of course, you would never want to downgrade your fluid. If your bike calls for DOT 4, use that fluid at minimum.
DOT 5.1 fluid is a further upgrade from the previous two. While using the same basic composition as DOT 4, it offers enhanced performance compared to the other two. As with DOT 4, you can certainly use DOT 5.1 as an upgrade when you are bleeding your braking system. The benefit is even higher boiling temperatures in both “dry” and “wet” states. Unfortunately, it is more hygroscopic than DOT 4, meaning you’ll have to replace it more frequently if you choose it.
DOT 5 is the odd duck of the braking fluid world. Unless you have one of the relatively rare vehicles which specifically calls for DOT 5, you would do well to avoid it entirely. Unlike the other levels, this one is based on silicone and as such does not mix with any other types. It has some advantages over the others, including functionality in extreme cold as it was engineered for military applications. This is way beyond what we would ever require and if your bike has ABS, DOT 5 will actually be a liability.
How to Change Brake Fluid
The most basic method of changing fluid uses just a pair of tools, a hose, and a container. Armed with only those four items, you can change your brake fluid just as well as anyone. It will take a bit more time and effort, but you can do it. We will go over that method as well as what to do if you mess up and accidentally introduce air into your braking system. It happens, but it’s certainly not the end of the world and you can fix it yourself if it happens. We do want to note that bikes with ABS like our Speed Twin can require access to the ECU to purge the ABS pump. As that can vary greatly from bike to bike, we do not address that in our article.
Just the Basics
With a fresh container of fluid, an 8mm wrench, a Phillips head screwdriver, a length of preferably clear hose with an internal diameter of 0.25”, and a container, you can do this yourself. Here are all the steps.
Wrap a towel around your brake fluid reservoir to cover all plastic and painted surfaces. Brake fluid will quickly etch into them.
Using a Phillips Screwdriver, remove the reservoir cap and diaphragm and set aside.
Start at the brake caliper farthest from your reservoir, attaching one end of your clear hose to the bleeder valve and placing the other end in a container.
Use your 8mm wrench to loosen the bleeder valve one half turn.
Pull back on your brake lever and do not release it. Brake fluid will begin to flow through the hose. Note the slightly amber color.
Keep an eye on your reservoir. As the fluid approaches the outlet, tighten the bleeder valve. Now you can release the lever without fear of introducing air into the system through the valve.
Top off the reservoir. and repeat steps 4-6 until the fluid coming out of the hose is the same color as your new fluid.
Move to the caliper closer to the reservoir and repeat steps 4-7. This should take less fluid as you are really only changing the fluid within that caliper.
Repeat all steps for the rear braking system.
Check brake levers and pedals to ensure they are firm. If they’re not, you’ve made a mistake and your bike is unsafe to ride until this problem is corrected. But don’t worry, we got you.
With Better Tools
To make the job easier, there are a couple extra tools that can help you out. If you already have a rear stand, by all means use it. This will keep your reservoir closer to level so you don’t have to fill it as many times, and it will reduce the chances of spilling fluid on your bike. The next tool is somewhat universal. A brake bleeder vacuum pump will eliminate the whole brake lever procedure so you can simply drain, fill and repeat until finished. Better yet, you can also use this with your car’s braking system so you can do it solo. Furthermore, it contains all that fluid in a sturdy container you won’t have to worry about spilling all about. We like this one from Griot’s Garage and this Mityvac unit on Amazon. Note: we are not affiliates of either company.
All the items needed to quickly and efficiently change the brake fluid on our Speed Twin
Wrap a towel around your brake fluid reservoir to cover all plastic and painted surfaces. Brake fluid will quickly etch into them.
Using a Phillips Screwdriver, remove the reservoir cap and diaphragm and set aside.
Start at the brake caliper farthest from your reservoir, attaching one end of your vacuum bleeder to the bleeder valve, making sure your vacuum valve is closed, and creating a vacuum in the canister.
Use your 8mm wrench to loosen the bleeder valve one half turn.
Open the vacuum valve. Brake fluid will begin to flow through the hose and into the container. Note the slightly amber color.
Keep an eye on your reservoir. As the fluid approaches the outlet, close the valve on your vacuum pump.
Top off the reservoir. and repeat steps 4-6 until the fluid coming out of the hose is the same color as your new fluid.
Move to the caliper closer to the reservoir and repeat steps 4-7. This should take less fluid as you are really only changing the fluid within that caliper.
Repeat all steps for the rear braking system.
Check brake levers and pedals to ensure they are firm. If they’re not, you’ve made a mistake and your bike is unsafe to ride until this problem is corrected. But don’t worry, we got you.
What if I messed Up and My Brakes are Squishy?
This can happen if you’ve accidentally let go of the brake lever with a valve open or if you’ve replaced a braking component. Don’t worry, we have all been there. Now you’re into the realm of not just replacing fluid, but “bleeding” them, a term for purging air from the system. If you find yourself in this situation, go back and check that your bleeder valves have all been tightened down. Then go to the farthest caliper from the reservoir and repeat the following steps until there are no longer bubbles in your fluid and the lever/pedal are firm. Attach the hose to your bleeder valve, pull and hold down the brake lever/pedal. When fluid slows down, tighten the bleeder valve, then let go of the lever. Repeat as often as necessary, making sure not to let the fluid in the reservoir fall below the drain port.
Always be certain to check your brake lever and pedal after working on your braking system.
Time to Do It Yourself
Now that you know how to do it, get out there and make it happen. We like to do it just before the riding season begins so we know we have the freshest brake fluid possible.
