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Silicone brake fluid was all the rage 20 years ago when I got my car, and that's what was in the car when I got it.

Gary Hall in particular was a big advocate of it.

The theory behind silicone fluid made sense to me, so despite dire predictions of horrible things happening if I didn't change it over, I never did. Mainly because of having to totally disassemble and rebuild the entire braking system.

So today I checked the reservoir and bled a bit of fluid out of one of the rear calipers. Both areas showed crystal clear, colorless fluid with no debris.
If this had been glycol fluid in there all that time it would have been brown mud that came out.

I probably would not choose silicone fluid at this point if I had to rebuild my brake system anyway, but I see no reason to change at this point. If 20 years isn't a good test, what is?
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Originally posted by comp2:
Since silicone is does not absorb water; it settles. The point which needs to be checked is the low spot where the water settles out; the calipers.

Water from where? Glycol fluid absorbs water from the atmosphere, but silicone does not.
If the system was cleaned of glycol fluid (or fresh anhydrous fluid was present in the system) before the silicone fluid was added, where would the water come from?
Water is present in all fluid in suspension. One of silicones first application in race cars was the hill climbers. They liked it because of the high boiling point. They soon found it's pitfalls. While normal brake fluid can absorb water, silicone cannot. What this means is every drop in suspension which exist from day one settles out in the low spots.

That single drop settles int he calipers and if the calipers get hot enough, that one drop boils rendering the brakes useless. Pretty much any competition if silicone is used the practice is to flush it before every race.

A non-silicon hygroscopic fluid can absorb around a table spoon of water without loosing one's brakes. A silicone fluid can loose it's brakes with as much as one drop in the system.

I don't compare it to regular brake fluid in how long it looks good. With regular brake fluid most manuals tell you it should be replaced in a 2yr minimum yet when is the last time any of us changed brake fluid. Yet silicon is recommended before any performance event or yearly at minimum.

It's great for collector cars and parade cars for the reasons you state but that is why guys with performance cars avoid it.
Condensation can form in brake systems just like anywhere else. It may not be much but it does accumulate. Reading through my handy dandy brake systems book it says that as long as the vehicle is not equipped with an antilock system (as silicone fluid can foam if pumped through small orfices) and it has been completely cleared of glycol fluids it should be fine.

Last edited by panteraturbo
Things not so rosy in silicone fluid land after all:

I have disassembled all 4 rear calipers.

The two original ones were pretty healthy, although one piston in each was pretty stiff and probably not contributing to braking much.

All the pistons in the 'additonal" calipers were frozen to one degree or other. One outboard "direct" piston won't even move at all with hitting the piston from the inside with a hammer with the caliper in a vise , using a piece of pipe to avoid hitting the locking gizmo. I think that can safely be called frozen.

So I rebuilt the original calipers and all the pistons move with thumb pressure.

It seems to me that the "extra" calipers were just adding drag and no braking at all.

My take on it is that the original calipers were probably disassembed when the switch to silicone fluid was made while the "extra" ones were not. In fact, based on the circumferential corrosion in the bores, they may have been corroded when they were installed. Had they had glycol fluid in the dependent portions of the bores only that area would have rusted.

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