SILICONE BRAKE FLUIDS – A WORD OF CAUTION
AP RACING NEITHER MARKETS SUCH FLUIDS NOR RECOMMENDS THEIR USE WITH OUR OWN OR ANY OTHER BRAKING SYSTEM
Virtually all of the problems with Silicone Brake Fluids 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°C 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 exacerbates 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 Racing glycol based fluids do not contain the adverse properties described above. The recently introduced Formula DOT 5.1 which exceeds the performance criteria of DOT 5 (Silicone), is suitable for all conditions likely to be encountered in modern driving conditions.