In the July / August issue I had a letter published updating a previous article on water pumps, I also suggested that past features should be reviewed. This has prompted me to write a series of articles reminding (or in some cases reading for the first time) members of technical tips and recommendations from old magazines. I will also add some of the problems, and I hope solutions, given to me as one of the ‘technical advisors’. Overhearing conversations at the last ‘International Weekend’ I am prompted to make the following the first article as it is still current: -

 In the November ’93 letters Terry Lawson of Australia told us of the experience of a friend who put silicon brake fluid in his racing car but found he repeatedly lost braking efficiency during races. Terry, being a chemist, did some research into this and found that the particular silicon fluids that form the basis of the brake fluid composition are compressible (to a relative large degree in comparison to other liquids). So much so that these fluids were proposed to be used in shock absorbers in aircraft landing gear. I had a similar experience to Terry’s friend during some ‘parade’ laps at Silverstone, after some very vigorous braking I nearly went into the back of Phil Dawson’s Healey. Needless to say I was using silicone.

A few issues later John Pilling wrote this: - I changed the fluid in my FF 11 to silicone. I have never been completely happy with my car as it had a long spongy travel, and once, after much braking on a hot day, the brakes became nearly non-existent. I enquired about my braking difficulties to a brake specialist and he gave me the following service bulletin by ‘AP Lockheed’: -

‘Our technical service department is receiving an alarming number of calls from motorists reporting problems with silicon brake fluids. AP Lockheed neither markets 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 with us over many years and recently ratified in SAE publications, namely: 1) High ambient viscosity. 2) High air absorption. 3) High compressibility. 4) Low lubricity. 5) Immiscibility with water.

Research has shown that the relationships between problems reported and problems identified may be expressed as follows:

Long and 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 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.’

This prompts the question ‘why do people change to silicon brake fluid?’ The main reasons are that it is non-flammable, it will not discolour or take off your paint-work if leaked or spilt and it will not take in moisture. The first two reasons are very real. The most common cause of under-bonnet fires is from brake fluid being spilt directly onto the exhaust from reservoirs that have not had their tops put on correctly. The remedy is always be careful and cure any leaks especially around the master cylinder area. As for the paint stripping properties of glycol fluid, again be careful when filling and bleeding. The non-absorption of water claimed by silicon fluid manufacturers may be true but as we have read, water will still get in the system. To reduce the effects of this with both fluids is for regular changes of fluid.

I hope the above shows the disadvantages of silicon, I know members are still using it. The good news is that to change from silicon to glycol does not need a complete strip down and seal change only a flush through (the seals do not absorb the silicon), unlike going from glycol to silicon. Also the higher ‘Dot’ numbered glycol fluids now available are proving to be better than anything available before.

Alan Smith