My understanding was Dot 3 and Dot 4 (and it seems Dot 5.1 as in five point
one) are compatible. But some differences on what I found below was
interesting. I have run Motul RBF600 in a few cars and had good results with
it, also ran it in an external clutch slave in the Mustang, and running it in
the Tilton Hydro-TRB in the Cobra. The Cobra has not been in use yet so hard
to say if it would have any effect, but I would expect it not to have any
natural rubber seals, which might be more common on older british stuff.
<stolen from a discussion found on the web that seemed credible>
DOT3: Brake
Fluid Grade - Min dry boiling pt =205degC or 401degF. Min wet
boiling pt =
140degC or 284degF. aliphatic polyether-based, can be mixed
with DOT4,
characteristics will be (linearly) in between DOT3/DOt4
parameters. Cheap,
most widely used, eats paint, absorbs water easily, eats
natural rubber seals.
DOT4: Brake Fluid Grade - Min dry boiling pt=230degC or 446degreesF. Min
wet
boiling pt = 155degC or 311degreesF. borate ester based, can be mixed
with
DOT3, characteristics will be (linearly) in between DOT3/DOT4
parameters, 50%
more expensive than DOT3, eats paint, significantly
increased resistance to
moisture absorption compared to DOT3 (but still
absorbs water), does not eat
natural rubber seals.
DOT5.1: Brake Fluid Grade - Min dry boiling pt=260degC
or 500degF. Min wet
boiling pt = 180degC or 356degF. It's a stupid name as
it's glycol-based
(borate ester) like DOT3/4 and has NO silicone. Absorbs
water, eats paint,
more expensive than DOT4, difficult to locate.
DOT5: Brake
Fluid Grade - Min dry boiling pt=260degC or 500degF. Min wet
boiling pt =
180degC or 356degF. Cannot mix with other fluids as it
contains silicone,
turns to jelly if done so; colored purple. Used mostly
for antique cars and
some motorcycles because it does not damage paint and
does not absorb water
(it allows water to pool in the low parts of the
braking system). Is more
compressible than DOT3/4 though. Twice as
expensive as DOT4 (and thus 3 times
as expensive as DOT3) and has limited
availability.
Sandy
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