Chris, Tom, Ken,
Thanks for correcting me. You all are correct. The
silicone fluid-designated DOT 5 is not hygroscopic. Better yet,
it doesn't even mix with water. Since silicone fluid doesn't absorb
water, its wet boiling point is essentially the same as its dry
boiling point. It won't harm paint like the polyglycols (DOT 3 and 4).
DOT 5.1 is a completely different chemical.
Silicone brake systems can become contaminated with water,
however, even if silicone fluids can't. Moisture can enter the system
through the master cylinder vent cap. Since water is the heavier of
the two fluids, it sinks to the bottom of the system, usually ending
up in the caliper. There it sits until the temperature of the
surrounding fluid reaches 212 degrees F, at which point the water boils,
forming those pesky pockets of gas that reduce braking. The calipers
are usually the first points to corrode in DOT 5-filled systems.
At this point I'll also add another error in my email, saying
that DOT 5 was designed for racing. That is incorrect. DOT 5 was
designed for military vehicles that sit for a long time and then
have to run on a minutes notice. DOT 5 is not really suitable for
high-performance applications. Why? Silicone fluid's only real
drawback is that as it approaches its boiling point, it becomes
slightly compressible, resulting in a spongy brake feel. DOT 5 is
supplied standard in the braking systems of Harley Davidsons.
I also have to clarify things for our European readers.
There are no silicone-based fluids in common use in Europe, yet
the Europeans do use a "5" designation for a polyglycol based fluid.
I retract my earlier advice, and at least amend it by saying
that for performance situations, I would advise against DOT 5.
My apologies for the confusion and misinformation.
Shane Ingate in San Diego