Silicone brake fluid - swapping to it

[denviola]

I'm wanting to swap to silicone brake fluid, and now that I'm replacing the master cylinder, front calipers and rear wheel cylinders it should be the best time. I have used silicone fluid on my race cars and my son's quarter midgets exclusively. Just don't have any experience in street cars that had existing brake fluid. I'm thinking a good flush will be enough, and wanted to see what others have done.
BTW this is being done on my 87 GLHS. I'm wondering specifically about the proportioning valve block, and old fluid in that. Is silicone fluid not a good idea in these cars?
Thanks in advance.

[Holley]

Really, its best to replace all unions etc. Silicone isn't really suitable for road cars, it doesn't absorb water, so the water collects in joints etc and leads to a springy pedal quickly. Interesting that you use it in racecars, as all the racecars I've seen up close enough to know (SCSA stock cars and a Lola) use DOT 5.1, for higher heat tolerance.

[BadAssPerformance]

I believe DOT 5 is silicone? If you flush out all the DOT 3 it should be ok.

One of the local guys runs it in pretty much every car he owns from his '56 Dodge D500 to his Turbo Mini.

[denviola]

Really, its best to replace all unions etc. Silicone isn't really suitable for road cars, it doesn't absorb water, so the water collects in joints etc and leads to a springy pedal quickly. Interesting that you use it in racecars, as all the racecars I've seen up close enough to know (SCSA stock cars and a Lola) use DOT 5.1, for higher heat tolerance.
I should clarify that the race cars I'm talking about were drag racing vehicles and were kept inside, as well as my son's quarter midgets. No weather to deal with. They only got used once per week, and didn't generate a lot of heat. Heat in brakes is what drives off moisture, if I'm thinking correctly. Used silicone to prevent the soft brakes, as condensation in original brake fluid is a system killer. I expect my GLHS will be mainly kept inside, except for events, autocrosses and some track use at a certain SRT&SDAC event this summer. It's not pretty on the outside yet, and I want 100% mechanical first.

[Holley]

DOT 5 is silicone, DOT 5.1 is the decent glycol stuff. Of course if you don't change your brake fluid every 18 months (or run DOT 3 or Cuban fluid, lol) a silicone system would feel tight in comparison.

SCSA cars arn't used in the wet either - they're like ASA cars, only with more power on tap ;) The brakes get very very hot (190 to 40 every lap of Rockingham UK), the silicone breaks down in the calipers in them and it comes out all lumpy after just one race. I'm lucky enough to live just down the road from one of the teams and know half the crew.

Water in the brake system will build up unless you keep your car in a moisture controlled bubble forever. With Glycol based fluids its absorbed and redistributes through the fluid to result in a consistent amount of compression over time, brake heat doesn't drive it off. With silicone it collects in nooks and crannies, which makes flushing very difficult.

BTW its easy to go from silicone to glycol, so you could always try it and then switch.