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Thanks once again to Gary Morrell. |
About ABS
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 With
the ignition off and the brakes not applied, there is little danger of
high pressure in the brake lines. There is, however, fluid under pressure
(about 1200 psi) in the ABS accumulator. To be safe, please disconnect
the battery while bleeding an ABS equipped vehicle. |
Why we bleed brakes
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First, brake fluid is hygroscopic, it absorbs water. Water
in the brake fluid lowers the fluid's boiling point and rusts the steel
parts of the brake system, which are many. Since the fluid is subject to
a great deal of heat in the calipers, we'd like the boiling point to be
as high as possible. Boiling brake fluid makes for really entertaining
(not) stops. Second, if the brake fluid has been boiled in the calipers,
say, due to some "spirited" driving, it is useless and needs to be removed. |
Selecting brake fluid
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Silicone brake fluid is the only fluid that is not hygroscopic,
however, the seals in many passenger car brake systems are not compatible
with silicone, and, at high temperatures, silicone becomes compressable,
making for mushy pedal feel. Because of this, silicone is not a racing
brake fluid, contrary to popular opinion. Silicone is commonly used by
vintage car restorers to preserve valuable brake systems. I do not recommend
it for the street or the track.
The fluid that is most at risk for water accumulation in the brake system
is the stuff in the plastic reservoir. Not only is it exposed to moisture
every time you open the cap, but the plastic reservoir is slightly water
permeable, so this fluid is constantly exposed to contamination. There
are 2 morals to this point:
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Before bleeding the brakes, steal the wife's turkey baster and use it to
suck the old fluid out of the reservoir. Refill it with fresh fluid and
then start bleeding. This way you're not wasting time bleeding old fluid
thru the system. Marital bliss hint: buy your own turkey baster.
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Don't buy brake fluid in plastic bottles: the manufacturer has thoughtfully
arranged for it to be water contaminated before you buy it. Buy brake fluid
in metal cans, Ford Heavy Duty DOT3 is used by more racing teams then I
can remember, and the cans are metal.
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General procedure
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Don't worry too much about changing the fluid that's in
the ABS accumulator. It is isolated enough that there's little danger of
water contamination.
Start by bleeding the longest line first, that would be the right rear,
then move to the driver's rear, then to the right front, and finally the
driver's front. Also, on Tauri that have the brake bias adjuster on the
rear suspension, make sure that the rear suspension is at normal ride height,
if the wheels are hanging, this adjuster reduces the flow to the rear calipers,
making the bleeding go really slow. |
Specific Techniques
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Several methods will be described here - One-man is not
my preferred method as there is greater danger that air will be
introduced into the system. Read on...
One-man method
For one-man you'll need a container that will stand on its own and a length
of tubing that will fit snugly on the bleeder fitting and reach the bottom
of the container. Connect the tubing to the bleeder fitting and fill the
container with enough fresh fluid to cover the end of the tubing. Now check
that the master cylinder reservoir is full. If you didn't empty it and
replace with fresh fluid, do it now. Open the bleeder fitting about 1/2
a turn, then climb in and SLOWLY pump and release the brakes about 10 times.
Slowly is important here, you're just trying to move fluid, not see how
far you can squirt it across the garage. Now check your bleed container
and top off the master cylinder reservoir again. DO NOT let the reservoir
get empty, it'll introduce a bunch of air into the system that will take
you forever to get out. After 20 pumps on each caliper, close the bleeder
fitting and make sure you can obtain a firm pedal. Then move to the next
caliper.
Two-man method
The bleed hardware is the same but you'll need one man on the pedal and
one at the caliper. The caliper man opens the bleeder and says "Down",
the pedal guy slowly pushes the pedal to the floor and HOLDS IT THERE,
the caliper guy then closes the bleeder and says "Up", the pedal guy then
releases the pedal. Repeat 20 times for each caliper, don't forget to top
off the reservoir occasionally. The beauty of the two-man method is that
there's no danger of pulling air into the caliper around the bleeder fitting
threads when the pedal is released. Now you can split a six-pack with your
assistant and do some of that male bonding stuff.
Power bleeding
Power bleeding, not to be confused with power lunches or power ties, is
where a canister of brake fluid under pressure (about 5 psi), is connected
to the master cylinder reservoir and then each bleeder fitting is opened
for awhile. The reservations I have with this method are that the fluid
is of unknown origin and God knows how long its been in the canister and
what its been exposed to. The pressure for the canister is usually provided
by the garage's shop air which is normally full of water anyway. Not recommended.
Phoenix Injector
The Phoenix Injector is a "sophisticated hand pump capable of directly
pumping liquids and gases (air) at pressures from -20" Hg to 150 PSI."
According to the manufacturer "The Phoenix Injector can bleed brakes and
clutches quickly and easily-- one man, no compressed air or electricity".
See their website for more details.
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