At last I have found some info that clarifies the brake fluid numbering
system, and I thought I'd relay it to the list..
to summarize:
DOT 3, DOT 4 and DOT 5.1 are all glycol (mineral?) based fluids.
DOT 5 is silicone and coloured purple to distinguish it. Unless that is you
are in Europe, which apparently does not recognise silicone fluids, so
european DOT 5 is glycol based and is the same rating as DOT 5.1 elsewhere.
To prevent this confusion, silicone DOT 5 is usually refered to as just
that.. "DOT 5 silicone"
DOT 5.1 does not necessarily have a higher boiling point than DOT 4. It was
developed to be lower viscosity and thus better suited to ABS braking
systems. Check the labels for boiling points.
The main advantages of DOT 5 silicone are that it doesn't destroy paint, and
it is not hydroscopic (water absorbing).
Modern glycol based fluids have initial (dry) boiling points similar to DOT
5 silicone, and in some cases even higher. However, as they get hot then
cool, condensation occurs and water is absorbed (i.e. they are hydroscopic),
and the boiling point is gradually lowered. Thus the need to replace glycol
based fluids regularly if used in racing etc.
DOT 5 silicone is not recommended for racing as it gives a spongy pedal due
to its basic structure. It also turns into a gel when very hot.
Never mix silicone and glycol based, as it will coagulate into a gooey
mixture. Changing over from one base type to another is a major job as all
traces of the previous fluid must be removed.
I feel much better after all this.
Greg
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