Brakes - Silicone brake fluid, to use or not to use that is the question!

28 September 2000

Over more recent years I've had affair number of discussions on the use of silicone brake fluid as opposed to the more common type. I have to say some folk are vehemence in their belief that this stuff is 'the answer' to all kinds of brake maladies. Some time ago I contacted the two major brake specialists in this country (UK) to see what they had to say. Their reactions was as vehemence as those believers above. For what it's worth, I thought I'd spread the word according to the manufacturers - which happens to be similar to my limited experience with this 'liquid saviour'.

Some has heralded silicone-based brake fluid as being the last word in brake fluid, yet also brings a look of horror onto the faces of some. The classic car folk swear by it, particularly for vehicles that are not used much where brake seal failure can be experienced, and because it does not affect paintwork. A number of racers go pale at the thought of using it. So what’s the score?

A majority of the problems suffered relate to three main areas - long or spongy pedal feel, sudden loss of brakes (ulp!), and brakes ‘hanging on’. SAE publications have ratified these symptoms by identifying certain properties prevalent in silicone brake fluids. These have been named as high ambient viscosity, high air absorption, high compressibility, low lubricity, and immiscibility with water (failure to mix with water). It would seem that there are definite relationships between these properties and symptoms, and can be categorized as follows -

LONG/SPONGY PEDAL - the compressibility of the silicone-based fluid is up to three times that of the more common glycol-based types, so needs more pedal travel to actuate. Its viscosity is twice that of the glycol-based fluid - meaning it is thicker. This equates to slow fill rates that can trap air, and results in bleeding difficulties.

SUDDEN LOSS OF BRAKES - Entrapped air suffers gasification at relatively low temperatures, causing a vapour-lock effect. It’s immiscibility causes any free water caught in the system to boil at relatively low temperatures - producing a vapour-lock. Glycol-based fluids absorb water (the water dissolves into the fluid), and although this will reduce the boiling point, it is unlikely to cause severe vapour-lock at low temperatures.

HANGING ON - In disc brake systems, the sole mechanism for returning the pads to their normal ‘not in use’ position away from the disc is the tendency of the seals to recover to their 'at rest' attitude once the pedal is released. The low lubricity of the silicone-based fluid works against this recovery; it’s high viscosity amplifying this effect.

In conclusion then, just because silicone brake fluid is relatively expensive in comparison to its glycol-based counterpart - like fully synthetic high quality engine oils are in comparison to mineral oils - it does not mean that a higher performance will be achieved, as is the case for the oil. And just to underline this, neither of the major brake specialist companies in England produces silicone-based brake fluid. In fact they are most emphatic in their condemnation of it.

Curious, wouldn't you say?