Just found this on the MiniMania web site:
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BRAKES - Silicone brake fluid, to use or not to use - that is the question!
Over more recent years I've had affair number of discussions on the use
of silicone brake fluid as opposed to the more common type. I have to
say some folk are vehement in their belief that this stuff is 'the
answer' to all kinds of brake maladies. Some time ago I contacted the
two major brake specialists in this country (UK) to see what they had
to say. Their reactions were as vehement as those believers above. For
what it's worth, I thought I'd spread the word according to the
manufacturers - which happens to be similar to my limited experience
with this 'liquid savior'.
Some have heralded silicone-based brake fluid as being the last word
in brake fluid, yet also brings a look of horror onto the faces of
some. The classic car folk swear by it, particularly for vehicles
that are not used much where brake seal failure can be experienced,
and because it does not affect paintwork. A number of racers go pale
at the thought of using it. So what's the score?
A majority of the problems suffered relate to three main areas - long
or spongy pedal feel, sudden loss of brakes (ulp!), and brakes 'hanging
on'. SAE publications have ratified these symptoms by identifying
certain properties prevalent in silicone brake fluids. These have been
named as high ambient viscosity, high air absorption, high compressibility,
low lubricity, and immiscibility with water (failure to mix with water).
It would seem that there are definite relationships between these properties
and symptoms, and can be categorized as follows -
LONG/SPONGY PEDAL - the compressibility of the silicone-based fluid is
up to three times that of the more common glycol-based types, so needs
more pedal travel to actuate. Its viscosity is twice that of the
glycol-based fluid - meaning it is thicker. This equates to slow fill
rates that can trap air, and results in bleeding difficulties.
SUDDEN LOSS OF BRAKES - Entrapped air suffers gasification at relatively
low temperatures, causing a vapour-lock effect. It's immiscibility causes
any free water caught in the system to boil at relatively low temperatures -
producing a vapour-lock. Glycol-based fluids absorb water (the water
dissolves into the fluid), and although this will reduce the boiling
point, it is unlikely to cause severe vapour-lock at low temperatures.
HANGING ON - In disc brake systems, the sole mechanism for returning the
pads to their normal 'not in use' position away from the disc is the
tendency of the seals to recover to their 'at rest' attitude once the
pedal is released. The low lubricity of the silicone-based fluid works
against this recovery; it's high viscosity amplifying this effect.
In conclusion then, just because silicone brake fluid is relatively
expensive in comparison to its glycol-based counterpart - like fully
synthetic high quality engine oils are in comparison to mineral oils -
it does not mean that a higher performance will be achieved, as is the
case for the oil. And just to underline this, neither of the major
brake specialist companies in England produces silicone-based brake
fluid. In fact they are most emphatic in their condemnation of it.
Curious, wouldn't you say?
Keith Calver
Article Date: Sep 29, 2000
Car Accociations: MGB,MINI,MORRIS,SPRITE,TRIUMPH
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--
Doug Mitchell
mailto:dbmitch@peoplepc.com
'73 Spitfire 1500
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