Chris K. sez:
DOT 5 silicon fluid does not adsorb (adsorb should be the correct word -
not absorb) much water, unlike DOT 3 or 4 (which has a max of ~5-6%
adsorbtion, if I remeber correctly). Water is not not miscible in the
silicone fluids, whereas the glycol/polyglycol fluids (which are usually
the DOT3s) and the borate esters (usually the DOT4s) are very hygroscopic.
This difference in how the two types of fluids handle water leads to some
interesting behavior, from what I have read. Basically, the non-silicone
fluids spread the water through themselves, reducing the overall boiling
point of the fluid. In a brake system with silicone fluid the water will
tend to gather together into water bubbles, in various places in the system,
usually near the source of the water intake, like the rubber hoses at the
calipers. So while the brake fluid in the caliper will handle the heat, the
water bubble will not, turning to steam. Steam does not give a firm pedal
feel. Plus the water bubbles tend to concentrate corrosion.
I think the biggest reason one sees silicone based fluids in so many show cars
is the effect of non-silicone fluid on that show quality paint job.
mjb.
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