Silicone Brake Fluid

By Derek Stevson

(Reprinted with Courtesy MGCC "Safety Fast" November 1999)

Foremost among the problems of conventional glycol fluids, is their hygroscopic properties - when exposed to air, glycols absorb up to 12% water by volume. The water lowers the boiling point. Under normal driving conditions, a car, driven daily, or even weekly, will heat up the brake fluid and evaporate a significant amount of the absorbed water. At higher percentages of water the brake cylinders corrode rapidly leading to early brake failure (probably due to electrolytic action as the mixture conducts more). This occurs in cars that have not been driven for several months. On being driven, the rust or corrosion soon causes one or more seals to fail.

The other effect, lowered boiling point, does not usually bother the ordinary driver. It does, however, cause problems under extreme braking conditions, such as in racing at Le Mans where the brake discs can be seen to glow in the dark. Silicones were first used at the 12-Hour Endurance Race in the sixties on the Chaparral, an innovative ground effects race car. Most club racing never approaches this kind of heat load from braking.

The U.S. Army undertook a long term study of the problems of brake failure, primarily due to-corrosion, extreme heat (desert conditions), and extreme cold (arctic) conditions. They were experiencing an enormous maintenance problem. The average Army vehicle logs only about 2000 miles per year, with most of the mileage occurring in short bursts followed by long periods of inactivity. They found that, invariably, vehicles that had been sitting for several months had brake failures after being driven a few miles. They then evolved an elaborate scheme of draining, flushing and refilling the brake system with a non-hygroscopic preservative for storage, and then a similar reverse process for getting the vehicle ready for use. This was expensive and delayed 'readiness' but cost less than the continual overhauling of brake systems.

After extensive study they found that silicone fluids (DOT5) could be used instead. All U.S. Army vehicles with hydraulic brakes now use silicone with an attendant savings of in excess of $20 million per year over the flush-refill protocol. An additional benefit was the wider range of temperature extremes the silicone had over glycol based fluids. The U.S. Postal Service also adapted silicones for the same reasons.

Silicones do not attack natural rubbers and most synthetic rubbers and vinyl, and are often used as rubber vinyl preservatives (as in many car care products). In addition, it does not attack auto paints! The earlier U.S. version of D0T3 would destroy the natural rubber used in early Lockheed and Girling brakes from England. Only Girling "Crimson" and Lockheed "genuine" fluids were usable, and they were hard to get in the US. The later US D0T4 specification seemed to overcome this, as did the change to synthetic rubber for the seals. CASTROL-LMA (Low Moisture Absorption) did better than most.

DOT5 silicone fluid does not attack British made seals. An exception seems to be with some German manufactured synthetic seals (such as lately used on Mercedes). These compounds do react with silicones and more readily with ozone (the sidewalls of tyres seem to age more rapidly than those manufactured in other countries).

I have used DOT5 Silicone brake fluid in my 1962 MGB (both clutch & brake) since 1984 and in my TF 1500 since 1986. In both cases, I have added fluid only once since (a slight topping off). I have had no hydraulic problems with either car. On the other hand , I have had to replace or rebuild cylinders on my other cars filled with DOT4 glycol based fluid, invariably after the car was laid up for other reasons.

When I switched over to silicone on the MGB, I first attempted to simply flush of the system by adding silicone and bleeding the brakes. This sounds like it should work since the two don't mix (are immiscible), and was the method suggested by the supplier. I found, however, the following:- the old brake fluid contained a lot of water, which in contact with the silicone, separated into three layers: water, silicone, glycol. Ordinary bleeding procedures, designed to remove air, left water in the bottom of the wheel cylinders. A sure invitation to corrosion and failure in time. However the brakes work fine at first, since any "non-compressible" fluid or mixture will work. (See the Army emergency procedures for loss of coolant or hydraulic fluid under combat conditions, using readily available fluid from every soldier in the unit. Note that a complete overhaul is then necessary to minimise corrosion afterwards).

I have found that the best way to switch over is with a complete system overhaul. The brake lines should be flushed with acetone which dissolves the glycol gummy residues and removes any trapped moisture. The lines should be dried by blowing with air (ethyl alcohol can also be used, it is not as flammable and won't injure paint). All flexible lines and seals should be replaced so that all rubber that has been exposed to the glycol is removed. I think that the problems some have had with silicone may arise from not doing this. It may be that the problems come from the interaction of the different swelling agents in the silicone with those used in the glycol based fluid.

Some time ago, an article appeared about the dangers of using silicone. It was written by (I believe) a sales engineer for a company who markets DOT4 type fluid. The myths created by this have been widely quoted since. Among the myths is the fact that silicone is 3-times as compressible as glycol (TRUE) and that this leads to excessive pedal travel, such that the pedal will travel as much as 3cm further (FALSE). While it is 3 times as compressible, the compressibility is still a very small number. With the volume of our brake systems, the additional compressibility would at most add 0.1mm to the pedal travel. Most of the problems with "soft" pedal arise from air bubbles entrapped, and poor bleeding. One should take the precaution of pouring carefully so as not to introduce air bubbles. Wait for 10-15 minutes for any bubbles to escape, and then bleed the brakes.