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 the problems relate to long/spongy pedal, sudden loss of brakes and 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 and immiscibility with water.  Research has shown that the relationships between problems reported and properties identified may be expressed as follows:

Long/spongy pedal

Compressibility, up to three times that of glycol based fluids.

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

Air absorption - gasification of absorbed air at relatively low temperature produces vapour lock effect.

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 (100C or thereabouts).

Hanging-on of brakes

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.

High viscosity, exacerbating the above effect.

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 DOT5, and is suitable for all conditions likely to be encountered in modern driving.