What Is DOT In Brake Fluid?

Brake Fluid Type

Nature

Dry Boil Temp

Wet Boil Temp

Color

Common Uses

DOT 3

Glycol

401 F/205 C

284 F/140 C

Colorless to amber

Passenger cars and trucks

DOT 4

Glycol

446 F/230 C

311 F/155 C

Colorless to amber

Passenger cars and trucks

DOT 5

Silicone

500 F/260 C

356 F/180 C

Purple

Military and show vehicles

DOT 5.1

Glycol

518 F/270 C

374 F/190 C

Colorless to amber

Racing cars

[Source: AFCO, USDOT]

If you dig deep enough into any subject, you'll find controversy. Brake fluid's big issue is whether to use glycol- or silicone-based brake fluid. They don't play well together, so knowing which one to use is critical for the performance of your brakes.

On the previous page, we learned that your vehicle's brake fluid reservoir has instructions on what type of fluid to use. Brake fluid is referred to by its DOT number and can be any of four varieties, though most passenger cars in the United States take either DOT 3 or DOT 4. The DOT part refers to the U.S. Department of Transportation, which issues detailed standards for all motor vehicle brake fluids in the country. Federal Motor Vehicle Safety Standard No. 116 details three different classifications of glycol-based brake fluids, and one silicone-based brake fluid (SBBF) [source: USDOT]. DOT 5 uses silicone instead of glycol, which is now the standard for most military vehicles. For race cars and show cars, you'll want to go with this type because, unlike glycol, it won't eat the paint off the car.

Beyond that, what's the big difference between the four categories? The answer is twofold. First, there's the boiling point. The higher the DOT number, the higher the boiling point. Remember that temperatures at the can reach up to around 800 degrees Fahrenheit (426.7 degrees Celsius). While it's important to note that these high temperatures are only where the wheels make contact with the brake pad (not in the brake fluid itself), some of that heat is being transferred into the fluid, which needs to be stable in order to function.

The second part of the answer is about water. Glycol-based brake fluid is hygroscopic, meaning it readily absorbs moisture found in the air. This is good in the sense that you can't keep moisture out, so it's designed to absorb the small amounts that reach it and still do its job. The downside of this is that the more moisture in your brake fluid, the lower its boiling point. Silicone-based fluid does not absorb water, but rather separates out the denser water and lets it settle to the bottom of your master cylinder, leaving the brake fluid to operate in its pure form and keep its boiling point high.

Next, let's take a look under the hood to see how your brake fluid is doing.