I'm with you. I really can't see the point of changing to a silicone based fluid. DOT 5.1 has many of the advantages of DOT 5 IMO, without any of the potential problems. AIUI DOT 5 is not compatible with glycol based brake fluid, so unless a glycol based system is completely purged of any traces of the old fluid, any remaining 'could' cause problems. There could also be a problem with the seals themselves on an older system, as apart from the incompatibility of the fluid itself, early seals were not chemically resistant to silicone based fluid, meaning all the seals had to be changed for ones that were. Having said that, AFAIK all later manufactured seals are compatible with both types of fluid, but if the seals have been replaced with NOS seals it could still be worth checking. Mike.