I've bought two cars (Isetta and Bond Mk G) which had each been off the road for about 6 years, and both had had the system filled with silicone fluid. Both had dead brakes, as the master cylinder rubbers had become rigid - so didn't expand when under pressure, allowing the fluid straight past.
Research I did for the Isetta (around 1997) suggested that silicone fluid is great if you use the brakes regularly, but hopeless if left inactive for several years.
Incidentally, I fitted new master cylinders in both cases, flushed and refilled with "normal" fluid, and all was fine.