by Stu Brennan
I remember sitting in a tech session at a United several years back, listening to some of our experts talk about silicone brake fluid. They were describing how easy it was to put silicone fluid in–just fill the reservoir and bleed until the silicone fluid came out. I pointed out that there would still be a bit of old fluid left in the wheel cylinders and calipers, but no one thought that this would be a problem.
A recent answer to a technical question in ROAD & TRACK suggests that this is not the case. They state that if silicone (DOT 5) and glycol (DOT 3) fluids are mixed, the glycol will cause the rubber swelling additives to separate from the silicone fluid, and the silicone will cause the anti-corrosion additives to separate from the glycol.
So, it seems that the best way to change to silicone is to start with a completely dry system. The article also contains some interesting comments about the various types of rubber used in brake systems and how each fluid works with them.
See pages 199-200 of the November, 1989, issue.