July 2001
HYDRAULIC CLUTCH SYSTEM BLEEDING
By
Steve Grosekemper
When
Porsches' were first being designed and built, the clutches were actuated by
long braided steel cables. These cables worked fine for quite some time, but
as they aged, problems began to surface. The biggest problem was that as the
lubrication went away, the level of friction between the cable and the bowden
tube increased. This situation could cause a normally light clutch pedal to
be nearly impossible to actuate. The size and strength of the clutches also
increased causing a need for better, more precise actuation.
Then
with the introduction of the 928 in 1977 we saw the first hydraulically actuated
clutch system. With fluid being pushed through a steel line there was virtually
no friction in the system. The positive movement of fluid also nearly eliminated
the need for free play at the top of the pedal. Where once we had an inch of
free play we now have only a few millimeters.
This
system caught on quickly and soon all Porsches had hydraulic clutches. This
system carried on with the 944 in 1983 and the 911 in 1987. While this system
is nearly trouble free, there are a few potential problems we need to be aware
of. The main problem with these systems is proper bleeding. The proper way to
bleed a hydraulic clutch is to attach a high pressure bleeder to the brake reservoir
and force the air and fluid down out of the system.
The
biggest problem with this logic is that air wants to go up, not down! The answer
is really quite simple. The first thing you want to do is remove as much fluid
from the reservoir as possible. Then attach the pressure bleeder to the slave
cylinder bleed fitting. The pressurized brake fluid will now work with physics
and push the air back up into the clutch master cylinder and out into the reservoir.
But
what about those budget home mechanics who can't afford a top of the line $200
pressurized brake bleeder? Believe it or not, I have an answer for this as well.
Instead of using a fancy brake pressurizing unit, go the store and buy an oil
squirt can. Fill the can with brake fluid and connect a 3mm hose to the end.
Pump the oil can to get all the air out of the line and connect the other end
to the slave cylinder bleeder screw. Now manually pump the fluid up the line
to the reservoir and you will be back in business. Correct the reservoir level
after bleeding.
The
lines of the hydraulic system are very tight and twisty with lots of places
for air to hide. So don't feel bad if it takes a few tries to get all of the
air out of the system. Just keep pumping the fluid through the system being
sure to keep the oil can full and the reservoir empty!
Good Luck
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