What brake fluid is best? Which brake fluid should I be using? How often should I change it? These are common questions that come up almost daily in motorcycle communities everywhere. We’re going to tackle the answer in-depth right here, right now!
First off, check your bike.
Every OEM master cylinder cover (the cap over the bit you pour the fluid into) will tell you what kind of brake fluid your motorcycle was designed to use. If the cap/cover was replaced at some point with an aftermarket one, or it’s unreadable, your owner’s manual will tell you that information as well.
If it says DOT 3, DOT 4, or DOT 5.1, DOT 5.1 is the best choice in most cases. Read on to find out the difference!
About DOT 3, 4, and 5.1
DOT 3, 4, and 5.1 are all polyglycol-ether-based, meaning they are all very similar fluids. They are all more or less interchangeable, with one caveat: you can’t go down in numbers. They are all champagne-colored, and they will all remove paint pretty quickly if spilled on a painted surface. Clean up any spills ASAP and rinse immediately with plenty of water. What this means is, if your cap says DOT 4, you can use a DOT 4 or a DOT 5.1 fluid, but not a DOT 3. We’ll cover why that is when we get to discussing boiling points later in the article. Aside from boiling point, the viscosity of each fluid is another difference, though to a small degree.
What about DOT 5?
DOT 5 is a silicone-based fluid that is purple in color and is separate from the series of DOT 3, 4, and 5.1. It is immiscible (won’t mix) with water and other brake fluids. Because they’re incompatible, DOT 5 must not be combined with other fluids. It also must not be used in a system where other fluids may be present. Additionally, it can’t be used in vehicles with an ABS system. The rapid cycling of the ABS unit will aerate the fluid, causing brake failure. Being immiscible with water, DOT 5 doesn’t absorb moisture from the atmosphere, so it has a longer service life than 3/4/5.1. DOT 5 also won’t harm paint should you spill some on a painted surface, since it’s silicone based.
As you can see in the table above, DOT 5.1 has the highest minimum boiling point of the 3 glycol based formulas mentioned. Keep in mind that these are minimum boiling points, so it’s possible to find a DOT 4 fluid with a higher boiling point than listed. In fact, as we mentioned in the Best DOT 4 Brake Fluid article, some fluids like Motul RBF 660 have a much higher advertised dry boiling point.
Why are there 2 boiling points?
Wet Boiling Point is defined as the temperature DOT brake fluid will begin to boil after it has absorbed 3.7% water by volume. Dry Boiling Point is measured with uncontaminated fluid.
Why do brake fluid boiling points matter?
A motorcycle cruising down the road has a lot of kinetic energy, which is energy of motion. Your bike’s brakes work by turning that kinetic energy (motion) into heat, using friction. When you pull the brake lever, you push a piston in the master cylinder. That piston pushes against your brake fluid and creates hydraulic pressure. That hydraulic pressure transfers through the brake lines and into into other, wider cylinders positioned next to the brakes on each wheel. This causes the pistons inside your brake caliper to “pinch” your brake rotor, using the brake pads to create that friction. Friction generates heat. Your brake rotor absorbs and stores a lot of that heat, but some of it transfers back through the pads and piston, into the brake fluid and caliper. If the brake fluid is exposed to more heat than it can handle, it boils and releases gasses. Since those gasses (like air) can be compressed, while fluids (like brake fluid) can’t, this leads to having soft, weak feeling brakes at best, and zero stopping ability at worst.
How does brake fluid get “wet”?
Glycol-ether (DOT 3, 4, and 5.1) brake fluids are hygroscopic (water absorbing), which means they absorb moisture from the atmosphere under normal humidity levels. The absorption rate is ~1.35% by volume per year, meaning it hits the “wet” range and must be flushed about every 2 years, on average.
Why is DOT 5 called DOT 5 and not DOT 5.1 or something else entirely?
Honestly, I’m not sure why they got confusing with the names. DOT 3, 4, and 5 should have been the Glycol based line of fluids, with the less-used Silicone fluid having a new name structure (DOT 5.1, for example) if you ask me. After all, it was developed mainly with performance/collector vehicles, race cars, and military vehicles in mind. (Most passenger & commuter vehicles have ABS systems, thus can’t use it.)
When should I flush my brake fluid?
If you buy a used motorcycle, flushing the brake fluid when you first get it home is never a bad idea. Not sure if or when your brake fluid was flushed? Probably a good idea to go ahead and do it, then. Ditto if you’ve noticed your brakes feel spongy and/or weaker than they used to be. Most factory manuals recommend a brake fluid flush every 2 years or 24-thousand miles, anyway. If the service manual does not include this information, make sure to check your brake fluid once a year. Check the color of the fluid as well as the moisture content. A cheap brake fluid tester (like this one) will give you a usable moisture content reading. As for color, that part is fairly easy.
Dot 3/4/5.1 get cloudy & dark with age as they absorb moisture over time. It may even turn brown or green as the moisture level builds up and begin to interact with different metal and rubber components in the brake system. Since DOT 5 doesn’t really absorb moisture, it typically turns lighter in color, eventually looking more like the new DOT 3.
Are we ever going to cover the viscosity difference mentioned earlier?
Absolutely! The other difference between DOT 3, 4, and 5.1 is a slight change in viscosity. (for simplicity’s sake, we’ll call this it’s “thickness”: for example, syrup has a higher viscosity than water.) The average motorcyclist won’t be able to tell a difference from one viscosity to the next when it comes to braking performance, but there is one exception. If your bike specifies DOT 4 LV (Low Viscosity), and you live in a colder climate, you probably want to keep using DOT 4 LV or Pentosin LV, rather than switch to 5.1. While 5.1 is a low viscosity brake fluid, 4 LV is slightly lower still. This slight difference will keep your brakes working better in the cold. The fluid passages in a system designed for DOT 4 LV sometimes have problems with other brake fluids, usually when it’s cold. Better safe than sorry when it comes to your brakes!
Well, I think that’s everything. We already covered the best DOT 4 brake fluids in another article, and we’ll be covering the best DOT 5 and DOT 5.1 choices in additional articles, so check back for those later!
Have questions or additional information? Leave a comment below, and we’ll get back to you ASAP!