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Correct brake fluid CM400C

rebrands

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Aug 24, 2021
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Location
Colorado Springs, Colorado
I recently did a front brake rebuild (new stainless steel pistons, seals and boots). I put in all fresh fluid, using DOT3, then read in the fine print on the bottle "High Temperature Synthetic". I've always shied away from synthetic brake fluid, believing it is incompatible with conventional fluid. But after looking I find synthetic is practically the only stuff on the shelf. I've not had any problems, but since the entire system was drained and refilled, maybe it's ok.

So, what's the deal? Has synthetic changed? Is is compatible now?
 
All brake fluids are synthetic even if they don't state it.

You are thinking of Silicone Based Fluid that doesn't play well with some brake systems or other types of brake fluid. Silicone brake fluids also are not suitable for ABS systems which has caused some issues with old time Harley guys who got new bikes with ABS but were used to using silicone fluids in their old bikes.

Personally I use Motul 5.1 brake fluid in my vehicles.
 
All conventional brake fluid is synthetic in nature. Silicone brake fluid DOT 5 should not be mixed with DOT 3, 4 or 5.1
Copied from https://www.machinerylubrication.com/Read/30614/know-brake-fluids
The chemical composition of the fluids also changes with the different classifications. DOT 3 fluids are glycol ether based.DOT 4 fluids are a mixture of glycol ether with borate ester. DOT 5.1 fluids use borate ester with glycol ether blended in, while DOT 5 fluids are silicone based.
 
I understood that DOT3 brake systems can be filled with DOT4, but should continue with DOT4 thereafter.

I looked up the difference once, DOT4 has all of the same properties as DOT3, but with a higher maximum operating temperature.
 
When I worked on the Harleys for the City of Tampa in the '70s, we switched all of them from DOT 3 to DOT 5 because the rear master cylinders were fading due to the heat of the front exhaust running right by the mounting location of the master cylinder near the right footboard. We simply flushed the master cylinders with DOT 5 repeatedly until the content was all fresh, clean DOT 5 and that was that (I was 22 years old, I had never heard of DOT 5 fluid at the time, did not know the implications of mixing the two and just did what I was told). But it worked just fine.
 
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