Silicone Brake Fluid - Beware
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A comment by Brian Smart, Service Technical, AP Lockheed
We are often asked by members of classic car clubs similar to the TR Register, what are the benefits of Silicone brake fluid?
AP Lockheed neither markets such fluids nor recommends use with our own or any other braking system, unless specifically designed to use such fluid.
Virtually all problems relate to :
-Long/spongy pedal
-Sudden loss of brakes
-Hanging of brakes
They reflect certain properties of silocone fluids (advocated) 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 problemes reported and properties identified may be expressed as follows:-
Long/sponge 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 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*C or thereabouts).
Hanging on of brakes
a) Low lubricity. In disc brake system [in] the sole mechanism for normalization of system pressure upon release of pedal pressure is a designed -in tendency of (the) 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 SupremeDOT5.1 which exceeds the performance criteria of DOT 5 is siutable for all conditions likely to be encountered in modern driving conditions.
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This comment by AP does not necessarily represent the opinion of the author, for further reading about brake fluids check our German webpage.However, we felt that this information would be valuable to many owners of classic cars or at least it would be siutable for discussion.
Source:TR action 154 May 1999, p27