Brake fluid is a topic in which just knowing the basic concepts and facts is enough to make a very informed decision when selecting your next purchase. There are some universal properties that all brake fluids meet in order to comply with government regulations. The United States Department of Transportation (USDOT or DOT) regulations for brake fluid are the most widely recognized. They follow the FMVSS Title 49 CFR Part 571.116 regulation which can be found in its entirety here.
Brake fluids come in a couple different varieties and knowing the difference can be important. Modern brake fluids are numbered according to the DOT as DOT 3, DOT 4, DOT 5 and DOT 5.1 depending on their chemical makeup and their performance. DOT 1 and DOT 2 are obsolete categories that are no longer made.
The most important thing to remember when selecting brake fluid is that DOT 3, DOT 4 and DOT 5.1 are all compatible with one another, however DOT 5 in not compatible with any other type and cannot be mixed with the other varieties.
DOT 5 and DOT 5.1 are the two that are probably the most confusing though. According to the DOT specification, DOT 5.1 is a recognized grade, but it does not have its own performance specification. DOT 5.1 is simply a brake fluid that meets DOT 5 performance levels but is chemically compatible to DOT 3 and DOT 4 fluids.
The dry boiling point or the Equilibrium Reflux Boiling Point (ERBP) is the temperature at which the brake fluid boils with no moisture present. The wet boiling point or Wet Equilibrium Boiling Point (WERBP) is the temperature at which the brake fluid boils with moisture contamination. The viscosities display the range of fluidity and the color can be used to quickly and easily discern which type of brake fluid you have. The chemical components are typical of the industry, but they are not necessarily regulated.
As you can see in Table 2, the DOT 3, DOT 4 and DOT 5 specifications increase in performance as they progress. The boiling point minimums increase and, with the exception of the -40°C viscosity of DOT 4, the low temperature viscosities decrease making the brake fluids more stable at wider temperature ranges.
DOT 3, DOT 4 and DOT 5.1 brake fluids all share two very important properties that distinguish them from DOT 5 fluids.
- They are hygroscopic meaning they absorb moisture from the air
- They are harmful to painted surfaces and will strip paint
The fact that they are hygroscopic is both good and bad. It is good because when the fluid absorbs moisture contamination, that water is not free and will not cause corrosion within the brake system. However, It is bad because the moisture that does enter the system contaminates the brake fluid and decreases its performance. This is why DOT 3, DOT 4 and DOT 5.1 brake fluids typically need to be changed for fresh fluid much more often than DOT 5 brake fluid. Changing the fluid restores braking performance until excessive moisture contamination occurs again.
DOT 5 brake fluid has the exact opposite results with regards to those two properties.
- DOT 5 brake fluid is hydrophobic and does not absorb moisture at all
- DOT 5 brake fluid is an inert silicone that will not harm painted surfaces
Because it does not absorb moisture and it does not harm paint, DOT 5 silicone base brake fluid (SBBF) is commonly the brake fluid of choice for collectors and restorers of antique motorcycles. This is because it is extremely stable for long idle periods, and it will not cause harm to the rest of the vehicle if it leaks.
On the other hand, being hydrophobic has its good and bad sides as well. Since it will not absorb moisture, any water that does manage to infiltrate the brake system will separate from the fluid and collect at the lowest point. Since water will displace the brake fluid wherever it is, the components exposed to the water will be susceptible to corrosion if the water is not removed. This is why brake systems using DOT 5 brake fluid need to be periodically purged at their lowest point to remove any moisture buildup. However, if in a very dry climate with little humidity, a DOT 5 system can remain maintenance free for a very long time.
One drawback to DOT 5 brake fluid is its chemical compatibility. Since SBBF is the less common of the two types of brake fluid, not all seals and elastomers are compatible with them and it is possible it could degrade certain rubber components. So if you are thinking of switching to a DOT 5 brake fluid, there are two main things to do.
- Make sure the previous DOT 3, DOT 4 or DOT 5.1 brake fluid is completely flushed and purged from the system to avoid contamination.
- Confirm that the rubber or plastic components of your brake system are compatible with silicone. If they are not, replace them with compatible parts to avoid brake failure.
Aside from the physical properties above that differentiate between the various grades of brake fluid, there are many other properties and tests they all must meet for safety and performance purposes. They include:
* DOT 5 SBBF exempt
** See FMVSs Title 49 CFR Part 571.116 section referenced for details
So, when comparing brake fluids, the most important properties to look at are the boiling points. A brake fluid with a higher ERBP will typically also have better high temperature stability (longer service life) barring excessive moisture contamination. With moisture in mind, the WERBP is a good indicator of the fluid’s stability and performance under stress and contamination. A combination of high ERBP and WERBP are what you want to find in a brake fluid though for long life and high performance and those values are usually made public by the manufacturers or printed right on the label, so they are easy to compare.
If you are riding in low temperature climates, then the low temperature viscosity may be something to compare, but most fluids are perfectly fine at extremely low temperatures so it is rarely a concern for most people.
One last important thing I want to say and reiterate is that the DOT specification does not require the compositions of DOT 3, DOT 4 or DOT 5.1 to be those typical chemistries listed above, but those are fairly standard across the industry as being the most efficient fluids to use in order to meet the performance requirements. So you may come across some chemical variations, but they should all still be compatible with one another. DOT 5 as mentioned above is always silicone though and is never compatible with the other types. Happy riding and safe stopping!