I recently got an education on this.
I was confusing Silicone & Synthetic, like some others.
DOT 5 is silicone. Non-hygroscopic, does not mix with DOT 3, 4, or 5.1
(makes green goo). Becomes compressible when hot. SCCA used to ban this in
racing applications. Great for infrequently-used vehicles, as someone else
noted, because the hydraulic system doesn't turn to rust just sitting there.
I used it in a '66 Mustang with manual disk brakes, pedal was like a rock.
Also in a Lancia Scorpion, from which I was never able to purge all the
glycol, so I had Green Goo. The is was an autocross car, but braking was
never compromised. But it had sorry brakes to start with :)
DOT 5.1 is synthetic glycol, mixes with DOT 3 & 4, but has a higher boiling
point.
I have no experience with it, but the dry boiling point was 550 as I recall,
which makes it suitable for racing applications. Had a spirited email
exchange just this week with a Valvoline tech who kept insisting that WET
boiling point was the only significant number. I'll probably try it as soon
as I run out of Perf Friction Z-rated.
Rex Burkheimer
Fort Worth TX
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