This tip comes from Joe Way of Sierra Specialty Automotive
Most modern brake rubber is made of EPDM, and there is definitely no problem with it and silicone. The currently available Spridget rebuild kits, packaged in the clear plastic bags with the white labels, are made by a British company called Nelson-Stokes using a proprietary rubber compound. So far as I know, there is no problem with their rubber and silicone.
The issue is that silicone fluid is not nearly as good a lubricant as glycol fluid. In a brake system this is not really a problem since once the system is bled the pistons move only a fraction of an inch at each application, and the applications are relatively infrequent. But a clutch system is of a type sometimes referred to as "long-travel" because the pistons travel through their full stroke at every application, and the clutch is typically used much more frequently than the brakes. So there is a LOT of piston movement in the clutch, and the poor lubricity of silicone will cause accelerated wear of seals, bore and pistons.
Silicone is specifically NOT recommended for clutch systems, and there is of course no way to separate the two systems in a Spridget dual cylinder.
Go to Disc Brake upgrade article