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[T3] DOT3 v. DOT5

All of the brake light switch comments reminded me of
the huge controversy on other mailing lists over
silicone (DOT5) v. non-silicone brake fluids.

Back in my motorcycle racing days (er, 15 years ago
now, how time flies), anyone on the mailing
lists/newsgroups of the day would have been horrified
by the idea of using DOT5.  Much more recently (1-2
years ago), I found that most people in the old
British car field are horrified by the idea of NOT
using DOT5.  Many religious flame wars ensued over
this issue.  As usual, the truth is somewhere in the
middle, and it's something of a horses for courses

DOT3 and DOT4 are mineral-based brake fluids.  DOT5 is
pretty much always silicone-based.  There's also a
DOT5.1 which is apparently mineral-based or fully
synthetic, but NOT silicone-based.  There are also a
good many other fluids which are off-the-chart in
terms of performance, but legally fall into either the
DOT4 or DOT5 classifications.

Mineral-based fluids are relatively thin, and are
hydroscopic.  They like to attract and retain water. 
Over time (months to years, depending on climate),
enough water will collect in these fluids to lower
their performance (they boil at lower temps, and
boiling fluids compress easily, so they lose their
ability to transmit force from the master cylinder to
the slave cylinders effectively).  All that water also
tends to cause lots of internal rust on any iron or
steel brake components (the British particularly are
fond of cast-iron in brake parts).  They're also very
nasty to paint.  Get a drop of mineral brake fluid on
paint it and will eat it up very quickly.

Silicone-based brake fluids are relatively thick, and
not hydroscopic.  They're not destructive to paint. 
Because they don't attract water, they don't lose
their effectiveness nearly so quickly, and they don't
promote rust in brake parts.

Sounds all rosy for silicone, yes?  There's a catch,
of course.  Silicone, being fairly thick, aerates very
easily.  There are nearly always many small bubbles in
the fluid.  Getting ALL of the air out of the system
is very, very hard.  Air is very bad in fluid as it
expands greatly under heat.  It also compresses very
easily.  So, while DOT4 will eventually lose its
effectiveness, the anti-DOT5 types insist that DOT5
doesn't work well out of the box (er, bottle).

Most of the people who don't like DOT5 are people who
insist on very high performance from their brakes
(racers, generally).  Most of the poeple who like DOT5
are people who let their cars sit for weeks undriven. 
Both are right, but often can't seem to see the logic
of the other side.

Oh, and if there are any plans to switch, mixing
mineral and silicone fluids results in a goo that's
nearly impossible to remove, and will happily block
the small passages in many brake systems.  You need to
use lots of brake cleaner and flush out the braking
system, or use all new parts, when switching from one
fluid to another. 

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