Silicon brake fluids - BEWARE

Isn't it strange how things work out, almost as if they are ordained.

Recently I decided I needed to look into why the Riley breaks needed pumping in order for them to work properly, after investigation I was advised to change the flexible hoses, apparently they tend to become a bit soft. I then decided to get the Espace back on the road so that I could take my time sorting the brakes out and to start work on repairing the doors. I found that the seals on the rear slaves had started leaking and the peddle was staying down after you applied the breaks , effectively causing the front discs to bind. The rear brake slaves were replaced but this did not sort out the peddle, further investigation was needed.

I finally came across some information in an old AA car maintenance cause book printed in 1983, it referred to swollen seals caused by using the wrong type of brake fluid. Unable to find any specific reference in the Renault handbook or Haynes manual, I was left stumped, that is until the latest copy of the Riley Record came through the door. Following a very good article by the editor detailing the extensive investigations he had carried out, was the following statement issued by Lockheed, (I don't know how old this statement is, and can't be held responsible for it's accuracy, so please make your own judgment):

"Our technical services 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, Hanging on of brakes.

They reflect certain properties of silicon fluids observed 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 relationship between problems reported and properties identified may be expressed as follows:

Long/spongy pedal
a) Compressibility up to 3 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, hence bleeding difficulties.
Sudden loss of brakes
a) Air absorption. Gasification of absorbed air at relatively low temperature produces vapour lock effect.
b) Immicibility (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 all boil and produce vapour lock at much lower temperature.
Hanging on of brakes
a) Low lubicity in disc brake systems, the sole mechanism for normalisation of system pressure upon release of pedal pressure, is a designed-in tenancy 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 on hard driving or even normal road use.

AP Lockheed glycol based fluids do not contain the adverse properties described above. Supreme DOT 5.1, which exceeds the performance criteria of DOT 5, is suitable for all conditions likely to be encountered in modern driving conditions."