Hydraulic fluid question

Originally posted by Barry Churcher
Good choice Bruce. Silicone fluid is not hygroscopic so will not hold moisture in suspension. What it will do though is separate out moisture and let it settle on the bottom of the cylinder pitting it worse. Silicone will hold air bubbles and they do not compress well, hence it is sometimes hard to get an adequate pedal. At the garage we will not put silicone in a system we have overhauled. I can put a guarantee on a Dot 3 system. For the antique seldom used vehicles we often work on we suggest bleeding the system each spring. This is an easy one man job with one of the vacuum bleeders and $3.00 worth of fluid.

Barry has stated almost exactly the warning statements well publicised by AP Lockheed that are on the net and copied verbatim below.

Further, AP L are forthright that they do not make or recommend silicone fluid and also state compressibility and lubrication problems with silicone fluid under tests.

It seems regular brake fluid is hydroscopic by design to entrain any condensed water throughout the system volume. Silicone fluids do not have this property and so water will settle, but worse than corrosion if a small droplet finds its way to a wheel cylinder or worse, a disc piston and then boils with conducted brake heat making a large volume of steam, the pedal goes to the floor and there are no brakes. Regular fluid will do this too when its hydroscopic capacity is finally exceeded.

Its noted that all vehicles now and for some years back have a fluid change schedule every 3 years to ensure entrained water levels cannot approach dangerous limits.

AP Lockheed had this to say:

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
o Compressibility, up to three times that of glycol based fluids.
o 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
o Air absorption - gasification of absorbed air at relatively low temperature produces vapour lock effect.
o 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
o 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.
o High viscocity, 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, is suitable for all conditions likely to be encountered in modern driving conditions.

R.