Brake fluid is a central part of your brake system. It lubricates the internal mechanisms of the system and transfers the force your foot exerts on the pedal into the clamping force that stops your vehicle. For this reason, standards are put in place to ensure that the fluid meets key criteria and allows your brake system to work as designed.
In Part 1 of this series, we discussed how kinematic viscosity and the boiling point (dry and wet) of brake fluid affects the system’s performance. In this article, we’ll be discussing the different Standards and Grades of brake fluid to give you a better understanding of what they really mean.
- Part 1: Key Characteristics
- Part 2: The Standards (this article)
The requirements for brake fluid is defined by a few internationally recognized standards organizations.
- The Federal Motor Vehicle Safety Standards (FMVSS) in the US, classifies three main grades of brake fluids under FMVSS 116. These grades are DOT 3, DOT 4, and DOT 5. DOT 5 if further defined as DOT 5 – Silicone Based Brake Fluid (SBBF) or DOT 5.1 – Non-silicone Base.
- The International Standards Organization (ISO) classifies a few other grades of brake fluids under their ISO 4925 document: Class 3, Class 4, Class 5.1, and Class 6.
- The Society of Automotive Engineers (SAE) classifies another three grades: SAE J1703, SAE J1704 and SAE J1705.
Each of these organizations define the minimum, maximum, and acceptable range of the key characteristics of brake fluid grade (such as the ERBP, wet-ERBP, and viscosities). They also go as far as defining the required packaging of the fluid to ensure no contamination occurs before it’s actually used.
You’ll realize that most of these organizations have different names for the grades/classes. Most of us in North America will probably be familiar with “DOT 3” or “DOT 4” brake fluid as defined by the FMVSS 116 standard. While the different standards offer different names, they tend to mostly all overlap and agree.
Here’s a table that shows the specifications of each grade.
You’ll notice that the table has columns named after the FMVSS 116 definitions of the grades except for the DOT 4+ column. The FMVSS defines DOT 3, DOT 4, DOT 5, and DOT 5.1 but It does not define the DOT 4+, the Super DOT 4, nor the DOT 4 – Class 6.
Technically, this “sub-grade” (let’s call it Super DOT 4) of brake fluid is a DOT 4 fluid. It meets all the requirements of a DOT 4 grade but has better viscosity characteristics. The Super DOT 4 is specifically the ISO 4925, Class 6 grade.
Boiling points for each fluid are shown in the illustration above. You’ll notice that DOT 5, DOT 5.1 grades have the highest boiling points followed by DOT 4 (and its sub-grades) and then DOT 3.
Viscosity of the brake fluid is measured at two points. The first point is at 100 degrees Celsius and the second point is at minus 40 degrees Celsius.
At 100˚C (212˚F) most fluids tend to be free flowing and it’s relatively easy to achieve a kinematic viscosity of 1.5mm2/s. At sub-zero temperatures of -40˚C (-40˚F), the fluids tend to “thicken” and it becomes difficult to ensure that it can still flow freely. Most modern vehicles have advanced braking control and traction control and will need to use this technology at frigid temperatures so this characteristic is extremely important in such environment.
For this reason, among others, most of modern vehicles will use a DOT 4 or Super DOT 4 grade since these grades address the need for low-temperature viscosity aimed at reducing ABS cycle response times.
In the good old days, brake fluid (DOT and DOT 2) was a castor oil-based fluid that was use for lubrication as well as for actuation. These days, most brake fluids are glycol-ether based fluids. The exception is DOT 5 which is a silicone-based fluid.
DOT 5 was initially developed for environments where the fluid’s water resistance and low corrosion was important – such as military or marine. The downside of the DOT 5 silicone-based fluid is it’s high compressibility when compared to the other grades. As such, it’s use-case is limited to very specific applications.
Why have colour requirements? The simplest reason is to ensure you’re using the right fluid. All ether-based fluids are clear to amber while all silicone-based fluids are purple.
While the different colours give a quick indication as to the composition, the clarity can tell how contaminated the fluid is. Ideally, brake fluid should be clear and clean. Dark and murky fluid indicates that the fluid has collected dirt, debris, and even moisture from the system and it’s now time to replace it.
Are the Fluids Interchangeable?
Vehicle systems are designed for to use a specific fluid grade. Ideally, the vehicle will continue to use that grade for the remainder of its life. The reason being that the internal components of the brake system have been designed, selected, and tested to work with that fluid. Although DOT 3 and DOT 4 are all ether-based, the chemical composition of each fluid grade is different and will have a different effect on your system. To ensure your system works as it’s designed to work, the best thing to do is stick to the manufacturer’s suggested brake fluid(s).
Can you use a Super DOT 4 grade instead of a DOT 4 grade? NOT ALWAYS. Your vehicle’s brake system would have seals, gaskets, valves, and sensors that have been designed to be used within a range of viscosities and temperatures. Using a Super DOT 4 in a DOT 4 system may present the system with a fluid that is too “thin” to work correctly with the existing equipment. Check with your dealership or owner’s manual for a clear answer on this.
Summing It All Up
Brake fluid is defined by a few internationally recognized standards organizations to ensure consistency in the performance of these brake fluids. These organizations and grades are:
- The Federal Motor Vehicle Safety Standards (FMVSS) under FMVSS Standard No. 116 defines grades DOT 3, DOT 4, DOT 5, and DOT 5.1.
- The International Standards Organization (ISO) under ISO 4925 defines Class 3, Class 4, Class 5.1, and Class 6 brake fluids.
- The Society of Automotive Engineers (SAE) classifies the grades SAE J1703, SAE J1704 and SAE J1705.