That squeal you hear in the middle of a
race isn’t from delight.
I BOILED MY BRAKES
I boiled my brakes at a race last
weekend. There were a lot of tight
turns, and I overused the rear brake.
Three laps into the moto, my brake
pedal went to the ground. When the
bike cooled off, the rear brake came
back and I raced the second moto
with no problems. Do I have any-
thing to worry about?
Yes. Just like tires, pistons, brake
pads and chains, brake fluid can
only go through so many heat cycles
before it loses its effectiveness. The
worst thing you can do to brake
fluid is to boil it. That not only chemically changes the fluid’s make-up,
but since brake fluid is hygroscopic,
which means that it absorbs water
from the atmosphere, boiling the
brake fluid releases gases. These
gases expand, and as they expand,
they decrease the pressure in the
hydraulic brake lines. You must
bleed the brakes every time the fluid
boils—regardless of what the brake
fluid looks like.
Brake fluid is a magic elixir.
To work properly, it must be thin
enough to respond instantly, compatible with brake system components
(plastic, metal and rubber) and able
to remain consistent in temperatures
that vary from 500 degrees to freezing. Brake fluid must stay consistent
from hot to cold and have the ability
to lubricate the pistons and seals.
Polyglycol fits the brake fluid profile
to a T. Unfortunately, glycol, an alcohol derivative, has one nasty habit. It
absorbs water. If you filled a coffee
cup to the brim with DOT 3 brake
fluid and sat it on your kitchen table,
over time the brake fluid would
overflow the cup’s brim. Why? Brake
fluid absorbs moisture out of the
atmosphere at an alarming rate.
Hydraulic brake systems are
sealed, so how does water get in?
There are three major culprits: (1)
Hydroscopic fluids, like brake fluid,
can actually pull moisture past the
master cylinder’s diaphragm. ( 2)
Moisture can enter when the reservoir cover is off. ( 3) The brake fluid
can be contaminated with water
before you put it in your brakes.
Although brake fluid is packaged in
a hermetically sealed environment,
it’s still not a good idea to purchase
a dusty container of fluid that looks
like it’s been sitting on the shelf for
years. Additionally, you should seal
the container immediately after use
to keep moisture retention at bay.
And, it’s best to throw open containers of brake fluid away every year.
Water absorption is a problem, but
so is heat. Using a few square inches
of brake puck to stop 400 pounds of
motorcycle and rider generates heat.
How much heat? Up to 500 degrees.
This heat is transferred through the
brake pads directly to the brake
fluid. Brake fluid can become so hot
that it will boil, and when it boils,
it vaporizes into a compressible gas.
compresses before the hydraulic fluid
and makes your brakes feel mushy.
If you mix the two troublemakers—
water and heat—things get worse.
When water gets in your brake fluid,
it lowers the boiling point and causes
the brakes to fade even earlier. This
dilemma is often called vapor lock, but
a more proper term would be vapor
fade, since the gas produced by the
boiling brake fluid rarely expands to
the point of locking the brakes.
What fluids are available to fight the
twin evils of water and heat? The DOT
(Department of Transportation), which
regulates brake fluid, has criteria so
stringent that only three companies
produce brake fluid in the USA. No
matter whose name is on the can, the
brake fluid comes from Dupont, Dow
or Union Carbide. The DOT grades
brake fluids by type, boiling point and
compatibility. Here are the big players
in brake fluid.
DOT 3: In almost all cases, DOT 3
is the least expensive and lowest-per-forming brake fluid produced. DOT 3
is compatible with DOT 4 and DOT
5.1 and has a wet-boiling point of 284
degrees. Note: The higher the wet
boiling point, the less susceptible the
fluid is to absorbing water.
DOT 4: DOT 4 is compatible with
DOT 3 and DOT 5.1 and has a wet
boiling point of 311 degrees.
DOT 5: The high boiling points of
DOT 5 make it look attractive, but if
water or air gets into the system, DOT
5 will get syrupy (not allowing the
micro bubbles to separate and float to
the top of the reservoir). DOT 5 is not
compatible with DOT 3, DOT 4 or DOT
5.1 and has a wet boiling point of 500
DOT 5.1: DOT 5.1 was designed for
the anti-lock braking systems (ABS)
on today’s automobiles. DOT 5.1 is
the thinnest brake fluid and offers the
least hot-to-cold change in viscosity.
DOT 5.1 is compatible with DOT 3 and
DOT 4 and has a wet-boiling point of
So, what should you use? First and
foremost, you should not use DOT
5. Repeat: Do not use DOT 5. Your
motorcycle most likely came stock
with low-cost DOT 3 fluid. After break
in, replace the stock fluid with new
DOT 3, 4 or 5.1 fluid. Be forewarned
that mixing new fluids with old fluids
lowers the boiling point of the new
fluid, so flush the complete system.
Although fluids with high wet ratings
can run longer before water contamination becomes too great, it’s never
smart to run fluid longer than six
months. DOT 3, 4 or 5.1 fluids all work
well. Your usage and standards determine which one is best for your bike.