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Hydraulic-assisted clutches
are found on many types of vehicles. Hydraulic
operation of clutch linkage is similar to the way
brakes operate. Liquids cannot be compressed, so
they can transmit motion and increase torque. The
input piston is located in the master cylinder
(action cylinder) connected to the clutch pedal.
A hydraulically operated
clutch.
It is located next to the
brake master cylinder. The output piston is located
in the reaction or actuator cylinder, commonly
called a slave cylinder. It is attached to the
release lever at the clutch. The two cylinders are
attached hydraulically by tubing and hose, just like
in the brake system. Hydraulic systems are popular
on custom vehicles because adaptation of the clutch
release mechanism is easy.
The only difference between a
clutch master cylinder and a single brake master
cylinder is that the brake master cylinder has a
residual check valve if it is for drum brakes. A
clutch master cylinder does not have one because the
clutch would stay applied as if a foot were always
resting on the pedal. This would result in failure
of a standard release bearing.
Visual Inspection
On vehicles with
hydraulic clutch linkage, check the clutch
master cylinder reservoir fluid level. It
should be approximately 1/4 inch (6.35 mm)
from the top of the reservoir. If it must be
refilled, use approved brake fluid. Also,
since the clutch master cylinder does not
consume fluid, check for leaks in the master
cylinder, connecting flexible line, and
slave cylinder, if the fluid is low.
Some diagnosis can be
done on the basis of pedal feel: A pulsating
pedal is due to something internal in the
clutch. The clutch will probably have to be
disassembled, but looseness or misalignment
somewhere could also be the culprit.
Air in the lines can
cause a spongy pedal on hydraulically
released cars. Bleed
the system to solve this problem.
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Slipping Clutch
A clutch is tested for
slipping by putting it in the highest gear
range (this would be fifth gear on a
five-speed). Set the parking brake firmly
and attempt to slip the clutch as if trying
to make the vehicle move. If the clutch is
slipping, engine rpm will rise. If the
clutch holds, the engine will die. There are
several possible reasons for a clutch to
slip.
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Partial Engagement
Partial engagement is
a common cause of a slipping clutch. In a
manually adjusted system as a clutch disc
wears, free play becomes less. Because of
the multiplication effect of the clutch
linkage parts, a small amount of wear on the
clutch disc can result in a total loss of
original free travel. If the wear progresses
beyond this point, it will be just as if the
driver is always "riding the
clutch" (pushing a small amount on the
clutch pedal). Slipping is the result. This
ruins the clutch disc (if it is not worn out
already). When a clutch slips, temperatures
inside the clutch housing can reach 500°F
in just a few seconds. Burn marks on the
pressure plate are one of the results.
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Partial
Disengagement
A cause of partial
disengagement is when there is a problem
with a hydraulic clutch release system. Look
for a low fluid level first. Then, check for
a failed master or slave cylinder.
Replacement of the leaking part fixes the
problem. Leaks in the hydraulic system can
be in hoses or lines but usually result from
age of internal rubber sealing parts.
- A leaking clutch or
brake master cylinder might be leaking
into the passenger compartment. Check
the carpet or floormat for evidence of
leakage.
- It is possible that
there might be no evidence of leakage.
This is because a master cylinder might
have an internal leak. The test for an
internal leak is to pump the clutch
pedal and hold it to the floor while
starting the engine. If the clutch
slowly begins to engage, there is an
internal leak.
A clutch master
cylinder has two rubber cup seals that can
fail. Courtesy of General Motors
Corporation, Service Technology Group.
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