Well said and clears up a lot of issues.
John Sims, BN6
[mailto:email@example.com] On Behalf Of
Sent: Tuesday, March 18, 2008 6:22 PM
Subject: Re: [Healeys] Synthetic, or other, brake fluid (sort of long).
Okay, folks, listen up:
Brake fluid is DIFFERENT than lubricating oil -- it is designed to resist
compressibility over a wide range of temperatures...that's what it does.
Nothing said below about brake fluid should be construed as applying to
lubricating oil (engine, tranny, or diff).
ALL brake fluid is synthetic. It may be made with a glycol base (dot 3,
or dot 5.1) or it may be made with a silicone base (dot 5).
Side note; That's "silicone," which is a slippery synthetic fluid, not
"silicon," which is sand and is used to make glass and microchips).
In order to make the fluids (glycol or silicone) compatible with the seals
(pretty much all synthetic rubber these days) so that when they interact the
fluid causes the seals to swell slightly and, well, seal, various additives
The additives in silicone-based fluids are different from those added to
glycol-based fluids. If silicone-based and glycol-based fluids are mixed,
additives cancel one another out (for chemical reasons that I don't
and they don't do what they're supposed to do to the seals.
Therefore, one should never mix silicone and glycol-based brake fluids --
that is to say, don't use Dot 5 to top up your reservoir if it was
filled with Dot 5.1 fluid (why they used almost the same numbers is
else I don't understand).
But you can use any combination of Dot 3, Dot 4, and Dot 5.1 if you have to.
Should you decide to change from glycol-based to silicone-based brake fluid,
if you want to be ultracareful, you should change your seals and rinse the
lines with alcohol.
None of the above, afaik, is open to debate.
The only thing that is open to debate is whether to use silicone-based or
glycol-based fluid. All I can tell you is that in a survey I did once, every
single curator or a major automobile museum I talked to used silicone-based
in every car in his or her collection. Also, no racer I've ever talked to
uses silicone-based fluid in a race car.
What is also not open to debate is that if you use glycol-based fluids, you
should change the fluid every two or three years. If you use silicone-base
fluid, it isn't necessary to change the fluid that often.
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