I asked some questions about changing to synthetic over DOT brake fluid and this is the answer that was sent my way. I didn't switch. Larry

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.