What does DOT mean when buying brake fluid?

Today we’ll be talking about the DOT number in Brake Fluid. Brake fluid is like blood in your vehicle’s brake system. Let’s start with a little history lesson. In 1924 hydraulic brakes were first used on mass-produced cars. They were alcohol or castor oil based, and they worked okay for those old cars, but their boiling point was too low for any modern cars. In modern cars, rarely do the rotors, and other brake components get heated above 204 degrees Celsius, but the boiling point of this old stuff was 190 degrees Celsius. So it could make it boil, and you don’t want your brake fluid boiling. You can say in a way your brake fluid boiling is more dangerous than your engine coolant boiling. Boiling brake fluid means the brakes will stop working. They will just fade away.

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DOT itself is short for Department Of Transportation. Numbers after the word DOT are a rating system. DOT 3 is a glycol ethers based brake fluid, and its boiling point is 205 degrees Celsius. The early 1924 brake fluids were categorized as DOT 2. You can’t buy that type of brake fluid anymore. Since the brake rotors rarely get above 204 degrees Celsius, DOT 3 is sufficient in most cases. But if you drive really hard, you might think about something else, like DOT 4 which is a glycol borate mix, and it can go to 230 degrees Celsius. So you are, never really going to have to worry about boiling brake fluid out.

stp-brake-fluid-dot-3Now many modern cars come with DOT 4 brake fluid. It’s top of the line for mass-produced cars. Now there is a DOT 5 fluid which is a silicone based fluid, but you can’t use this in most modern cars with ABS because it damages the ABS. The DOT 5 is silicon-based and is mainly used in military vehicles. It does not absorb water moisture, so they never have to worry about water moisture contaminating their vehicle’s braking system. But you really can’t use the DOT 5 in many cars. The silicone ones don’t mix with the other type of brake fluids. Like I said it can damage the ABS. It foams up and won’t work right so you can’t use DOT 5 on any car you’re driving on the street. However, there is now a DOT 5.1 fluid. It has the same boiling temperature as the silicone-based DOT 5, which is 260 degrees Celsius, but being a glycol borate mixture, it can be used with any of the other systems. You can put DOT 5.1 in DOT 4 or DOT 3, and you don’t have to flush anything out. It can work perfectly fine with any of them whereas you can’t mix with anything with DOT 5.

bosch brake fluid dot 4brake fluid dot 5finish-line-dot-5-1-brake-fluid

DOT 4 works perfectly fine for most cars, but if you want the absolute best with the highest boiling point, you can get DOT 5.1. However, it is always suggested to use the type of brake fluid which is recommended by your vehicle manufacturer instead of experimenting.

If you find this article helpful in choosing the right brake fluid, let us know in the comments section below.

Mohammad Ali

21st-century auto enthusiast and a student of automobiles. Blogger for an informative & productive Pakistan automobile community.

Notable Replies

  1. I guess its Department of Transport (USA) who graded these oils.

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