Re: [MV] brake fluid question

From: Richard Notton (Richard@fv623.demon.co.uk)
Date: Fri Mar 16 2001 - 23:49:05 PST


----- Original Message -----
From: "Everett Doyle" <194cbteng@pchnet.com>
To: "Military Vehicles Mailing List" <mil-veh@mil-veh.org>
Sent: Friday, March 16, 2001 1:55 PM
Subject: Re: [MV] brake fluid question

> John;
>
> I have been told by a fellow who has a job that would lend credence to his
> remarks that even if silicone will not absorb water it will retain water
in
> tiny drops much like the air bubbles you describe and then deposit the
water
> on nearest surface --i.e. inside wheel and master cylinders - there
causing
> the same rust damage as DOT#3 -- this was based on his experience with
> LandRover and Jaguar
>
>
>
> > I've had simillar problems, and I believe that the silicone brake fluid
> used
> > by the military holds tiny air bubbles. The smallest jarring of the
> > container (mine was a 1 gal. plastic jug) caused the fluid to be
> > "contaminated" with air. The problem is that it takes much longer for
the
> > air to rise up out of the silicone fluid. So, even while pouring the
> fluid
> > into the master, you've introduced air.
>
Silicone fluid is not all that it's hyped up to be, certainly it may be
totally inappropriate in a modern (disc braked) vehicle, especially in some
areas of the US with high ambient temperatures since its boiling point is
well below that of conventional fluid.

AP Lockheed has this to say:

AP Lockheed neither markets such fluids nor recommends their use with our
own or any other braking system.

Virtually all the problems relate to:
Long/spongy pedal
Sudden loss of brakes

They reflect certain properties of silicone fluids identified by us over
many years and recently ratified in SAE publications, namely;
High ambient viscosity
High air absorption
High compressibility
Low lubricity
Immiscibility with water

Research has shown that the relationships between problems reported and
properties identified may be expressed as follows:
Long/spongy pedal
a) Compressibility up to three times that of glycol based fluids
b) High viscosity up to twice that of glycol based fluids, leading to slow
rates of fill and retention of free air entrapped during filling and hence
bleeding difficulties.

Sudden loss of brakes:
a) Air absorption. Gasification of absorbed air at relatively low
temperatures produces vapour lock effect.
b) Immiscibility (failure to mix) with water. Whilst the presence of
dissolved water will reduce the boiling point of glycol based fluids, any
free air entrapped in silicone filled systems will boil and produce vapour
lock at much lower temperatures (100 deg C/212 deg F or thereabouts).

Hanging on of brakes:
a) Low lubricity. In disc brake systems the sole mechanism for
normalisation of system pressure upon release of pedal pressure is a
designed-in tendency of seals to recover to their "at rest" attitude. Low
lubricity works against this tendency.
b) High viscosity, exacerbating the effect of (a) above.

It should not be assumed therefore that the high price of silicone fluids
implies higher performance in hard driving or even normal road use.
AP Lockheed glycol based fluids do not contain the adverse properties
described above. The recently introduced Supreme DOT 5.1 which exceeds the
performance criteria of DOT 5, is suitable for all conditions likely to be
encountered in modern driving conditions.

End quote.

Reported problems with silicone fluid here are rife, the generally held view
is that it is applicable to undemanding applications (slow pre-war vehicles
with drum brakes) where the vehicle is used very infrequently.

Richard
Southampton - England



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