Brake fluid


Ken Chambers

> I know we discussed DOT 3 & DOT 4 brake fluid sometime last year but
> don't remember the 3 vs. 4 reasons as to which does what.

Here's a pretty recent explanation of brake fluids from our local San
Jose car guy columnist, Brad Bergholdt:

Clip (err, type)---
Traditional brake fluid can be a mixture of as many as 10 ingredients,
with the greatest percentage being glycol ether and polyglycol or
ethylene glycol (also used in radiator coolant). These ingredients and
others provide sliding lubrication, fluid viscosity, fluid boiling
point, rubber swelling control and corrosion/oxidation inhibitors.

Brake fluids of this type are hygroscopic, meaning they absorb water.
Oddly, this is desirable, because moisture that enters the system
spreads out, rather than creating a locaized blob that is susceptible
to freezing or boiling, and a source of corrosion. The downside is
that the boiling point of the fluid drops dramatically as water content
exceeds a couple of percent, and this can occur in as little as 18

A brake fluid's boiling point is important because some of the very
high temperature occuring in the brake system components is transferred
to the fluid. If the fluid were to boil within the calipers or lines
during heavy brake use, the result can range from a spongy pedal to
complete loss of operation. DOT-3 fluid has a minimum dry boiling
point of 401 degrees F and a wet (contaminated) boiling point of 284 F.
Heavy-duty DOT-4 fluid's numbers are 446 F dry and 311 F wet.

Silicone-based DOT-5 fluid has a higher boiling point (500F/356F) than
DOT-3 or 4 fluids and doesn't absorb water. It's also more resistant
to viscosity change under extreme temperatures. The drawback is this
fluid is more compressible because of air entrapment and isn't
compatible with residual DOT-3 or 4 fluid in the event of a fluid swap.
While popular with the U.S. Army and antique auto enthusiates, this
fluid is not recommended by any auto manufacturer, and should never be
used in an anti-lock brake (ABS) system.

What can you do to preserve/enhance brake fluid performance? There are
many newer "super" DOT-3 or 4 glycol based formulas offering boiling
points as high as 600 F. Be sure to stick with the DOT type
recommended by the vehicle manufacturer.

---Ken Chambers
'64 Red