No personal experience but this article from Moss motors makes a pretty good read on types brake fluid. I've copied a small snippet of converting to silicon brake fluid. The conclusion is that for a particular MC the effects are unknown due to not knowing the type of seal used. Follow the manufacturers recommendation.
http://www.mossmotors.com/SiteGraphics/Pages/Brake_Fluid/brake_fluid_long.htmlWhat About the Stories of Silicone Brake Fluid Causing Seals to swell?
These stories abound on the internet, but I have not actually spoken to the person it happened to, so I cannot comment on the actual incident that started the story. We do know that it is very, very unlikely that a brake fluid meeting FMVS 116 would do that. The testing is designed to make that sort of thing impossible. I can tell you that the literature is full of reference to the grotesquely swollen and gooey seals that you get if the brake system is contaminated with petroleum based oils or solvents. It does not take much of this kind of contamination to ruin all of the seals in a system. The contamination can usually be traced to the use of improper cleaners, rags contaminated with motor oil or grease, or handling the seal with hands that have petroleum based contaminants on them. We have tried to duplicate these reported problems by soaking brake cups in DOT 5 fluid, but we have never found a problem. However, these stories have a life of their own and it is doubtful that they will ever go away. It is perhaps a modern version of the traditional British natural rubber brake seal warning about using “the wrong fluid”.
Converting from Glycol to Silicone Fluid – The Decision
The decision to convert from Girling or Lockheed (or whatever you are using) brake fluid to silicone fluid is not a decision to make lightly. If you have been using a glycol based fluid and never had any issues, I suggest you stay with it. Implement a calendar based flush and fill routine to keep the brake fluid fresh and reduce the chance of a water contamination caused brake system failure. Regular fluid changes will also minimize the chance of corrosion damage. Talk to your mechanic about testing brake fluid if that appeals to you; just keep in mind that you should be testing for both copper and water in the brake fluid. If you are thinking of changing, the decision is not necessarily an easy one. With a modern car, the manufacturer specifies what you should use, and you can have confidence that the entire braking system has been designed with that specific brake fluid in mind. With our British classics, we don’t have that luxury. The specifications given when the cars were new are irrelevant. If you are considering a change, talk to members of your club. Find out what the people who have a car like yours are running. Spend some time on the various forums. If several people have made the switch to silicone and are happy with the results, you can seriously consider making the switch yourself. Why take these steps? All brake fluid, glycol or silicone based, must swell brake system seals to form tight seals and help prevent leaks. The maximum amount of swelling is also specified in the FMVS 116 testing. This defines a range of seal expansion that is “within tolerance.” This also means that different fluids can react with a given type of seal differently and still be within the specifications. Just because the fluid you are using is working fine does not mean another brand of brake fluid, even one that meets the same specification, will work just as well because it may not affect the seals in exactly the same way. In some master cylinders, for example, seals can swell so much that the relief port is blocked, which results in brakes that don’t release because the fluid cannot be pushed back into the master cylinders as the wheel cylinders retract. While brake fluid manufacturers will certify that their fluid meets BMW, Ford or GM specifications, they cannot be expected to say their fluid will work in an MG TD master cylinder.
Converting from Glycol to Silicone Fluid
If you do decide to convert to silicone fluid, it should be done as part of a total brake system overhaul, with freshly rebuilt or new calipers, wheel cylinders and master cylinder. Silicone fluid should not be added to a system that contains even small amounts of glycol fluid or contaminants. Merely bleeding the system is not enough, as there will be pockets of old fluid and sludge that will not bleed out. Silicone fluid tends to concentrate any residual glycol fluid, moisture and sludge into slugs instead of allowing their dispersal throughout the fluid, as does glycol fluid. This can lead to relatively severe but localized problems, rather than the more general system deterioration experienced with old moisture-laden glycol fluids. This may be a factor in reports of leakage when silicone fluid is used in non-rebuilt systems that had been operated with glycol fluid. A "new" system full of silicone fluid will require very little maintenance for years.
