Brake fluids, dots are different!

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. Please don't use it Scott, you'll hate it.

I did a little research on DOT 5.1. DOT 5.1 fluids are glycol based like most DOT3 & 4. 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 labeled 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 660F. 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.

                Dry Boiling Point       Wet Boiling Point
DOT 3           401 F                   284 F
DOT 4           446 F                   311 F
DOT 5           500 F                   356 F
DOT 5.1         518 F                   375 F