Re: Brake Fluid: Dot 3 vs Dot 4?

[Richard Beels <beels@technologist.com>]

Quick answer:  Use the DOT4.  Most racing brake fluid is DOT3 but has a
higher boiling point.  The water absorption is a big issue but race cars
have their brakes bled after every race.  Passenger cars usually only every
year or two....

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 pars 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.


At 10:25 PM 12/23/1998 ,  Dwayne Cosby was inspired to say:
> Quick question.
> 
> I see from the packaging that it is Dot 3. My S4 manual specifies Dot 4.
> What is the difference and what are the risks of using it?


Cheers,
	Richard
	88 90Q "Hannu" - K+N, new vac hoses, lubed U-Joint, still 0.0 bar....
	88 Golf GTi - PRO Rally