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Re: Brake fluids and rubber parts

To: "MG List" <>
Subject: Re: Brake fluids and rubber parts
From: Max Heim <>
Date: Mon, 30 Aug 1999 13:54:49 -0700
No, I think you are a little confused. *Silicone* brake fluid has never 
been accused of dissolving seals (or anything else). That is its chief 
benefit. DOT 3 or 4 brake fluid is notorious for dissolving paint 
everywhere it drips. It has also been known to dissolve the seals on 
LBCs, *except* for Castrol GT LMA brand fluid. Why is this? Who knows? 
DOT 3 and 4 are performance specs, not materials specs, so it is possible 
that Castrol uses a different formulation than Pep Boys, Mechanics Brand, 
etc. At any rate, there is plenty of evidence in on this subject, and the 
recommendation is clear:

Use exclusively Castrol GT LMA brand DOT 4 brake fluid, *or* completely 
flush your system and convert to silicone brake fluid (DOT 5, any brand), 
since silicone fluid is incompatible with the other variety.

Note that DOT 5 is not an "improved" spec compared to DOT 4, it is merely 
a different chemistry with different benefits. DOT 4 = higher boiling 
point; DOT 5 = non-corrosive.

Some people may not have had any trouble with "off-brand" DOT 4 fluid in 
their MGs -- if so, congratulations, but be aware that there are many 
different sources for brake system rubber components . The next time you 
rebuild, you might not be so lucky.

'nuff said!

James Nazarian Jr had this to say:

>I have had dot 5 (silicone) in my brakes and clutch of my mg for about 5
>years now and I haven't had a single problem.  My understanding is that
>the original and early replacement seals couldn't stand up to silicone,
>but that the newer replacement seals (within the past 5 years or so) are
>designed with a slightly different compound will have no problems with
>silicone.  I don't know the difference in the two kinds of rubber or
>fluids, but I would guess it is related to the same properties that make
>dot 3/4 take off paint and dot 5 prevent paint from bonding.  So far I
>haven't had any problems with the silicone, but I think there must be some
>truth to the rumor so I would make sure the seals in m/c and s/c are
>fairly new.
>James Nazarian
>'71 B roadster
>'74 BGT bastardization with big aluminum heart :)
>'63 Buick 215 cubic inch 'heart'
>On Sat, 28 Aug 1999, Larry Hoy wrote:
>> Ken Payne said (in part)
>> > I used AP550 for a while ... and [it] seemed to be harder on some seals.
>> So can anyone tell me about rubber seals?
>> What's the deal with certain brake fluids destroying certain fluids?
>> Is this problem a thing of the past?  Some how I gotta think that today's
>> brake rubber parts must be compatable with DOT 3, 4, and 5 brake fluids?
>> Can anyone enlighten me?
>> Larry Hoy


Max Heim
'66 MGB GHN3L76149
If you're near Mountain View, CA,
it's the red one with the silver bootlid.

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