Silicone brake fluid

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23,591
 Originally Posted By: edwardh1
can it be mixed with "regular" brake fluid?
As the other clown implies, why couldn't you? However, if you want your brakes to work, you must not mix silicone brake fluid with "regular" glycol-based brake fluid, because they are completely incompatible. Unless you plan murder. In that case, mix anything you want. You may mix the following brake fluids: DOT 3, DOT 4, DOT 5.1 Silicone brake fluid is DOT 5.0. You don't want it anyway. Cars that use it have lousy pedal feedback.
 
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21,663
Location
Apple Valley, California
6) Silicone fluid is functionally incompatible with systems that have held glycol-based fluids for any length of time, requiring flushing and seal replacement (there are counter opinions on this, which state that the modern silicone formulations are in fact compatible with only a flushing, rather than a complete reseal). The actual DOT specification requires chemical compatibility, so as far as that goes, the two fluids won't cause reactions if used in the same system, but they certainly won't mix, either.
 
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25,046
Location
ON, Canada eh?
 Originally Posted By: Chris142
6) Silicone fluid is functionally incompatible with systems that have held glycol-based fluids for any length of time, requiring flushing and seal replacement (there are counter opinions on this, which state that the modern silicone formulations are in fact compatible with only a flushing, rather than a complete reseal). The actual DOT specification requires chemical compatibility, so as far as that goes, the two fluids won't cause reactions if used in the same system, but they certainly won't mix, either.
Sounds like a bad marriage. "They won't cause reactions ... but they certainly won't mix either."
 
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12,385
Location
Northern CA
 Originally Posted By: StevieC
 Quote:
the two fluids won't cause reactions if used in the same system, but they certainly won't mix, either.
Sounds like a bad marriage. "They won't cause reactions ... but they certainly won't mix either."
I was an early user of DOT 5 in the early 1970s. Systems that were flushed less than perfectly had globules of one fluid floating in the other fluid. I never had any problems from a less than perfectly flushed system and a few others that I knew that used it didn't either. The mix just looks ugly. It was originally peddled as a high temperature high performance fluid, but as people gained experience (racers in particular) it became obvious that is was a turkey because it was springier than DOT 3-4 and gave a spongier pedal. Silicone fluid won't absorb water, that's true. What does happen is that if moisture gets into the system globules of water sit in the low spots and corrode spots worse than dissolved water in normal fluid does. There have also been reports of seal problems using DOT 3-4 seals with silicone. It doesn't seem to be a common problem and I didn't experience it. I think it might have been seals that were ready to fail anyway.
 
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23,591
 Originally Posted By: Steve S
When I took my ASE brake test they were compatible.
Did you pass the test (without cheating)? It must have been during the pre-ABS days. ;\)
 
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5,532
Location
Canada
Lots of info on Dot5 here. http://www.buckeyetriumphs.org/technical/Brakes/Fluid/Fluid.htm I have used it for 10 years in a collector car (silicone fluid does not attack paint) Once bled properly, you will NOT have a spongy brake pedal. Because this fluid tends to form micro bubbles, it can be necessary to re-bleed the system a day or two after the fluid is first added. I guess this is how the Myth of 'poor brake pedal feel' came about.
 
Messages
23,591
 Originally Posted By: expat
Because this fluid tends to form micro bubbles, it can be necessary to re-bleed the system a day or two after the fluid is first added. I guess this is how the Myth of 'poor brake pedal feel' came about.
It's why silicone fluid is not compatible with ABS brakes. At least I'm not aware of any car with ABS that specifies silicone brake fluid. As you said, it's great for a collector's car. No moisture absorbtion means decreased corrosion. And the clumsy collector won't ruin the finish of his Stutz with glycol-based brake fluid. As far as I know, silicone brake fluid is routinely used only in extreme climates.
 
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12,385
Location
Northern CA
 Originally Posted By: expat
Lots of info on Dot5 here. http://www.buckeyetriumphs.org/technical/Brakes/Fluid/Fluid.htm I have used it for 10 years in a collector car (silicone fluid does not attack paint) Once bled properly, you will NOT have a spongy brake pedal. Because this fluid tends to form micro bubbles, it can be necessary to re-bleed the system a day or two after the fluid is first added. I guess this is how the Myth of 'poor brake pedal feel' came about.
It's not a myth. The racers that were the first to figure out that is wasn't suitable for their application knew how to bleed barkes. Some of them also knew how to compare the bulk modulous of fluids. Check the numbers yourself. Your brake feel standards have to be fairly low to not be able to tell the difference.
 
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5,532
Location
Canada
Oops,I guess I should have re-read the article I posted! Dot 5 IS more compressible, and becomes more so with heat. In my application with a small, simple, braking system, the volume of fluid is small and the amount of sponginess at the pedal is insignificant (to me, undetectable for when I had Dot 4) But I guess with a large car with large calipers or in a racing application, Yes, there could be an issue. I stand corrected
 
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1,855
Location
Australia
 Originally Posted By: expat
I stand corrected
Lucky, I was about to jump on you from a great height Part of the reason why Castrol's SRF is so highly thought of in racing is it's lack of compressability compared to even poly glycol fluids at elevated temps (it's a silicon ester base, not to be confused with silicon DOT5 fluid ;\) )
 
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