An oft-overlooked but extremely important system on your motorcycle is, of course, the brakes. We’re going to talk about braking systems in a series of articles, and today’s fun involves brake fluid: what it does, why it’s there, and how to properly maintain it.

We all expect our brakes to work as advertised, and when they do we often don’t think about them beyond that point. If my experience in a motorcycle shop is any indicator, however, your motorcycle’s brakes need more attention than you are giving them.

Motorcycle brakes need regular maintenance; more so than a car does. Your motorcycle can eat a set of brake pads in just a few thousand miles. It is good practice to inspect your brakes as often as you check the air in your tires, which we all do at least every other week, right? Right, of course we do.

Your Motorcycle Owners Manual (MOM) suggests flushing the brake fluid at least every other year. Doing so more often will not harm your bike and may alert you to a hidden problem. More than once, I have witnessed when someone alerts a friend or acquaintance to the state of their brake fluid reservoir: the fluid is low, you should add some! Too often the conversation ends there, more brake fluid is added and everyone moves on with their lives.

Hold on! That’s not the whole story! Where did the brake fluid go? Well, into the system, that’s where. Motorcycle brakes contain a relatively small volume of fluid, and when the brake pads wear away (as they are designed to do), the pistons in the brake calipers extend farther out to keep the pads within easy contact of the brake discs. The more the pads wear, the more the piston extends. What’s extending that piston? Why, the hydraulic fluid in your braking system, of course. 

What Does It Mean?

When the fluid reservoir gets low, that’s a giant warning sign that your brake pads are used up. Take this opportunity to first replace your brake pads (which, when you push in the pistons, will in turn push the brake fluid back out of the caliper and into your brake system, making that fluid flow back up into the reservoir). Now that you have new pads installed, refresh that brake fluid. If you do things in the opposite order, you’ll find your pistons impossible to retract, or your reservoir spilling over.

Even if your brake pads are not used up yet, it might be time to replace the fluid in the system. You can see the fluid through the windows of your reservoirs, or, if they’re plastic, right through the reservoir walls; it should be the color of extremely pale clover honey. If it looks like tea, or worse, black coffee, replace that fluid! If your brake fluid is immediately dark even after a good flush, that’s a sign that your rubber brake lines are deteriorating. Look into replacing those lines.

Brakes: I've Seen Worse
Eh, I've Seen Worse.


Be careful with brake fluid; it is an excellent paint stripper. It will remove paint and mar finished surfaces in minutes. Whenever you work with brake fluid, cover everything that might get dripped on with a shop blanket or old bath towel, and have another damp towel at hand to clean up any spills. Wear nitrile gloves, since brake fluid is also toxic and terrible for your skin.

Prep Your Work Area

Set your handlebars so that the reservoir is as level as possible, and clean the cap thoroughly before removing it. Prepare a clean, stable surface to put the parts on (I use blue shop paper towels on my workbench). Note that under the lid there is sometimes a small, flat styrofoam float, and always a rubber accordion gasket. Be very careful with these. Note and remember their orientation, remove them and gently wipe all the old brake fluid off them, then set them aside somewhere clean and safe. Fill the reservoir as necessary as you bleed fluid out of the system, and seat the float and gasket very carefully on reinstallation.

Since there’s an absolute ton of videos knocking around the internet that will teach you to replace your brake fluid from here, we’ll say this: remember the rule of youtube—watch at least three videos since they’ll all have something different to say (and one might be off the rails). May I recommend one from Lemmy over at Revzilla, above? He knows what’s up.

DOT What?

He touches on a couple of important points, though, that I feel need a deeper explanation. The first one is brake fluid types and nomenclature. The lid on your brake fluid reservoir (which sometimes resembles a plastic “pee cup” like the one on Lemmy’s Triumph, and sometimes is square and metal with a round glass window in the side, and with two screws holding the cap on) has a bunch of warnings on it. One of them is “Use only DOT4 from a sealed container.” Yours may say the same but DOT3, or DOT5, or DOT5.1. 

Now this is important, and why the powers that be made it so confusing I do not know: DOT3, DOT4, and DOT5.1 are all glycol-based fluids. Each one has a higher mandated (but sometimes not actual) boiling point than the next. They are compatible and can be mixed without messing up your system, but for the best performance and peace of mind, stick with the fluid specified by your motorcycle manufacturer: the one that is stamped into your reservoir lid.

DOT5 brake fluid is silicone-based and should never be mixed with DOT3, DOT4 or DOT5.1. They are not compatible, will not mix, and can create a goobery mess, clogging up passages and resulting in sudden non-functional brakes. That obviously creates a very dangerous situation. Bleeding one into the other counts as mixing, so anything short of emptying your system completely, cleaning it with brake cleaner, and refilling from empty, will mix the fluids.

What About ABS?

Some motorcycles have ABS, and as such can have multiple bleed points. Often these brakes need to be bled in a specific order, including cycling the ABS module in between bleed points, so again, read your MOM or shop manual to find out what your motorcycle’s demands are. Bleeding brakes in the wrong order can cause ABS faults. If your ABS module gets bubbles in it or goes dry, it can lose its mind, and that can be a very expensive repair.

Use A Real Bleeder

While you can use any old clear plastic tubing for this job, I recommend buying a brake bleeder. It can be as simple as a purpose-made small plastic bottle with some tubes and a magnet, or slightly more expensive like the ever-popular MityVac, or a fancy compressor-assisted vacuum pump. Any of these, used according to the manufacturer’s directions, will help keep you from introducing air into your brakes’ hydraulic system. Air compresses, and bubbles will reduce or eliminate your brakes’ effectiveness.

Don't Just Trash It

Remember to dispose of your old brake fluid responsibly. Never reinstall old brake fluid; it is hygroscopic and repeated exposure to air (and humidity) will significantly reduce its efficacy by lowering its boiling point. Your friendly local auto parts supply shop can help you find out where to safely dispose of the old fluid.

Do you have more questions about brake fluid? What mistakes have you made when working on your brakes?

Source: Epic Bleed, How Stuff Works 

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