From: John Wise <60TR3A@cox.net>
>
> You need water for rust. Brake fluid is not water so it does not
> support the development of rust. However, traditional brake fluid is
> hydroscopic (sp?)
The word is hygroscopic.
> which means it absorbs water from the atmosphere.
> Overtime traditional brake fluid absorbs enough humidity that the
> water (which is heavier) ends up at the lowest part of the brake
> system & which supports the development of rust at those points where
> the water sits.
Not true, the water is fully absorbed into the brake fluid. It doesn't settle
to the bottom any more than it does in your whiskey bottle (pure alcohol is
also hygroscopic). So, the water makes all of the brake fluid corrosive, not
just the lowest points.
> The other down side is that when the brakes get very hot (e.g.,
> racing or very spirited driving) the water turns to steam which is
> compressible & then you get soft brakes.
Even "dry" brake fluid will boil, it just boils at a higher temperature than
"wet" brake fluid. And the usual result is no brakes at all ! The reduced
boiling point is just one of the many reasons that glycol (DOT 3/4/5.1) brake
fluid should be changed every few years. It turns out that glycol will absorb
not only moisture, but salt, right through the soft brake lines, so this is
recommended even on modern cars that keep outside air away from the brake
fluid. It absorbs even faster on Triumphs where the top of the MC reservoir is
open to the atmosphere.
Since silicone does not suck water out of the air (and even through soft brake
lines) like glycol does, it doesn't need to be changed as often. Has a higher
boiling point than DOT 4, too.
Randall
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