This was received from AP Lockheed, when someone queried them re: silicone
for street use.
GM
========================================
"SILICONE BRAKE FLUIDS - BEWARE
Our technical service department is receiving an alarming number of calls
from
motorists reporting problems with silicone fluids.
AP LOCKHEED NEITHER MARKETS SUCH FLUIDS NOR RECOMMENDS THEIR USE WITH OUR
OWN
OR ANY OTHER BRAKING SYSTEM.
Virtually all of the problems relate to:-
Long/spongy pedal
Sudden loss of brakes
Hanging on 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, 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
temperature
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
water entrapped in silicone-filled systems will boil and produce vapour lock
at much lower temperatures (100 0C or thereabouts).
Hanging on of brakes
(a) Low lubricity. In disc brake systems the sole mechanism for
normalisation
if 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 exceed the
performance criteria of DOT 5, is suitable for all conditions likely to be
encountered in modern driving conditions."
FAX from Dave Gaught (Brakes Tech)
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