Have been following this thread for a while. Some of the posts have been
good, other somewhat misleading. For instance, I have seen a couple that
said that DOT 3/4 and DOT 5 are compatible (they are not) and just saw
one that said DOT 3/4 and DOT 5 are 'miscible' (meaning they dissolve
in one another in all proportions). They do not and that is why they are
not compatible. DOT 3/4 fluids are high molecular weight glycol ethers.
Their high oxygen content makes them susceptible to hydrogen bonding and
because fo this, they tend to dissolve water (DOT 3/4 and water are
both 'polar' substances as well and DOT 3/4 is "hygroscopic" which means
that it abosrbs water). A silicon-based DOT 5 fluid is a relatively non-
polar substance that is more-or-less "hydrophobic' (doesn't like water).
'Like dissolves like' as we chemists say, and it shouldn't be surprising
that just as oil (non-polar, hydrophobic) floats on water (polar and
extensively hydrogen bonded), Silicon-based DOT 5 (non-polar, hydrophobic)
floats on DOT 3/4 (polar, subject to hydrogen bonding, dissolves water).
I learned this the hard way by topping up the master cylinder reservoir
on my 72 TR6 after I bought it with Castrol LMA (low moisture absorption)
DOT 4 fluid. What a mess! The PO had installed silicon-based DOT 5, but
didn't pass along this useful tidbit of info. Naturally, the system
had to disassembled, thoroughly cleaned and flushed to put things right
again. I still have the glass jug from the original flush with the
two distinct layers (DOT 5 on top, LMA DOT 4 on the bottom) sitting on
my shelf. Their physical properties as well as their densities are
quite different. Most auto restoration folks recommend silicon-based
fluids over DOT 3/4 because they don't absorb water. Water absorption
is what rots (rusts) out the steel brake lines of cars that sit a lot. I
don't know much about the performance of these fluids in racing, etc.,
but I went back to Castrol DOT 4 LMA fluid for my car. It's cheap,
plentiful, etc. As a final note, if you DO switch to silicon fluids in
your car, if you ever sell the beast, be sure to tell the new owner
about the swtich. He/she may be as dumb as I was in not checking
first before topping off the reservoirs.
Gary Morrow
Associate Professor of Chemistry
University of Dayton
300 College Park
Dayton, OH 45469-2357
(513)229-2048 FAX (513)229-2635
72 TR6, 68 TR250
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