Hello, Richard,
That's a valid point and not to be taken lightly, although most of the
literature that I've read suggests that this concern is significantly
greater for silicone-based fluids than for the glycol-based fluids that I
was discussing above.
With glycol-based brake fluids, water that finds its way into the fluid
becomes bound at the molecular level and tends to be evenly dispersed in the
fluid, which has the effect of lowering the fluid's boiling point. The more
water that is absorbed, the lower the boiling point. If the fluid has
absorbed say, 3 percent water, the boiling point will drop something like 25
percent, give or take a bit. With a glycol fluid like DOT 4, the boiling
point would be lowered from 446 degrees F down to approximately 300-310
degrees F. I've read several accounts that claim that 3 or 4 percent water
in a DOT 4 fluid is still safer than having globules of water in a DOT 5
silicone fluid. If you have a silicone fluid system with water globules
trapped in a wheel cylinder, those water blobs will have a boiling point of
only 212 degrees F, even though the silicone fluid itself has a far higher
boiling point. It would be quite easy for the water blobs to flash into
steam and cause spongy brakes or a total brake failure.
One might also want to research the expansion characteristics of water
itself vs. glycol fluid containing 3 or 4 percent water. I'd have to look
these up, but I wouldn't be surprised if water expands at a greater rate
when it turns to steam than does glycol fluid that has absorbed some water.
I could be wrong about this hunch, but if it's true, then a few small water
globules might create a larger steam pocket in a wheel cylinder when they
boil than would glycol fluid that has absorbed some water. Again, this last
bit is just conjecture...have to do some reading I guess.
Of course, you'd have even more protection with the glycol-based DOT 5.1
fluid (not to be confused with silicone DOT 5), assuming that it's
compatible with the brake system's parts.
Regards,
Alan
(Salt Lake City, Utah)
DOT 3 401 284
DOT 4 446 311
DOT 5 500 356 (silicone)
DOT 3 = 401 degrees F;
DOT 4 = 446
DOT 5 = 500
Dry Boiling point, deg F (minimum)
374
401
446
500
Wet boiling point, deg F (minimum)
284
311
356
DOT 2*
DOT 3
DOT 4
DOT 5
Dry Boiling point, deg F (minimum)
374
401
446
500
Wet boiling point, deg F (minimum)
284
311
356
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