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Brakes - DOT types

The Basics

"DOT brake fluid numbers refer to a set of minimum specifications that a brake fluid must meet. The two main specifications are wet and dry boiling points.

DOT 5 brake fluid is always silicone based, not by definition, but simply by the fact that it's the only base material to date that will meet the DOT 5 specs. There are good things and bad things about silicone brake fluid as with everything else in life. The good things are it won't eat paint and it won't absorb water. This generally means that it almost never needs to be replaced. The bad things are that it's rather viscous and tends to hold on to any air bubbles that may become entrained in it. Unless you work very carefully it is very hard to get a good pedal.

I did a little research on DOT 5.1. DOT 5.1 fluid is glycol based like most DOT3 and4. It will absorb water but it's not clear at what rate from the things I read. It has high wet and dry boiling points (see the table below). Some companies are selling DOT 5.1 fluids labelled as Super or Supreme DOT 4. This is probably a less confusing designation than the one chosen by the U.S. Department of Transportation since DOT 5.1 fluids show more similarity to DOT 4 fluids than DOT 5.

DOT 5.1 looks like it would be a good choice for street cars. Especially if it meets or exceeds DOT 4's hydroscopic specs. It's main advantage is it's high wet boiling point. For racers, wet boiling point shouldn't be a concern. Brake fluid should always be changed long before it can absorb enough water to lower it's boiling point.

In the U.S. most racers use a high performance DOT 3 fluid. Some of these have dry boiling points as high as 660 F. The race car brake systems are always flushed with new fluid before each race weekend. Sometimes even more often. DOT 5.1 should be a good off the shelf choice of racing but unlike DOT 5 it still requires periodic changes. It would be a shame to lose a race because you didn't want to spring for a quart of brake fluid."

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Brake Fluid Boiling points

Fluid type Dry Boiling Point Wet Boiling Point
DOT 3 401 F (205 C) 284 F (140 C)
DOT 4 446 F (230 C) 311 F (155 C)
DOT 5 500 F (260 C) 356 F (180 C)
DOT 5.1 518 F (270 C) 375 F (191 C)


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Brake Fluid URLs (added 20 Dec 2002 pl. report dead links) is an excellent guidance on DOTs and mixing... says:

"What is the difference between DOT3, DOT4, DOT5 and DOT5.1 brake fluids?

DOT 3, DOT 4 and DOT 5.1 brake fluids are glycol based compounds that are compatible with one another. DOT 5 brake fluid is silicone based and should never be mixed with DOT 3, DOT 4 or DOT5.1. DOT 3, DOT 4 and DOT 5.1 fluids may damage painted surfaces and DOT 3 and DOT 4 have lower boiling temperatures than DOT 5 (DOT 5.1 has the same boiling point as DOT 5).

Furthermore, DOT 3, DOT 4 and DOT 5.1 fluids are "hygroscopic", which means they absorb moisture from the air. This causes the fluid to turn dark, indicating that it is time for the brake fluid to be replaced. DOT 5 fluid will not damage paint, has a boiling temperature in excess of 500f F, and is not hygroscopic."

Also, try as suggested by the webmaster.

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silicon oil

Details: silicone oil specific gravity 1.115 ( assuming he means fluid) from Prakash

Added 16 DEC 2002

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07 May 2003