In the M3 I went from BMW brake fluid to SRF without doing that thorough a flush. Pretty sure I got all the old fluid out of the calipers and main lines, but I definitely didn't touch the ABS/DSC module at that time. Zero problems.Your call. It is exceptional stuff, but thorough flush is needed. I know some people who use it on track bcs. it allows them to run one season without change, but I know there were issues with it bcs. people didn’t want to flush al the way as it is expensive.
Been doing some more Googling and reading. Looks like a lot of people are seeing that SRF is based on "silicon ester" and getting that mixed up with "silicone" (as in DOT 5), and/or they think anything that outperforms DOT 4 by that much must be DOT 5, and/or they are over-generalizing from super-niche race-only brake fluids.
Either way, if there were any potential problems with mixing SRF with other DOT 3/4 brake fluids, Castrol would say something about that, no? They say the opposite -- i.e. that SRF is fully compatible with DOT 3/4 fluids and systems. The only problem they mention with failing to fully flush out the old fluid is that you won't get the best performance.
In theory, I can see the argument that SRF's reduced water absorption might reduce its service life in street use. If left in long enough, it might allow some water to accumulate and pool where another fluid would have just absorbed it. Maybe track people aren't seeing this because the high temps boil off any water that isn't absorbed, meaning the service life of the fluid is dictated mainly by how much water it absorbs, and thus SRF nets a longer service life than other fluids in those regimes even though it'd be unwise to run for too long on the street. IDK. That's some seriously underinformed spitballing on my part.
Either way, the flip side is that SRF would suffer less from sitting on the shelf because it's absorbing less moisture in that time.
So... maybe okay to use if it's just been sitting in a tighly capped bottle, but not for longer than a year in service? Would be interesting to find out. Maybe time to buy a moisture tester and some test strips...
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