Basic description: When you step on the pedal you push in on a
small piston in the clutch master cylinder, which pushes fluid out
of the cylinder, and causes a similar piston to move in the slave
cylinder. This in turn pushes a lever on the side of the
transmission. The lever pushes on the throw-out bearing, which
pushes on the springs on the pressure plate, and thus releases
pressure sandwiching the clutch disk between the pressure plate and
the flywheel. (whew! see diagram.) The flywheel and pressure plate
are bolted to the engine, and the clutch disk rides on the
transmission shaft. When you step on the pedal, the clutch disk
(and transmission shaft) can freewheel, turning independently from
the engine. Release the pedal, and everything "locks" together, and
the transmission input shaft turns at the same speed as the engine.
=== Slave Cylinder and lever
| | (some cars- return spring on lever)
| | O
--| | | \ |
engine |======]=[======= transmission shaft
--| | | / ^Throw out bearing
| |
| ^Pressure Plate
Flywheel^ ^Clutch disk
Note: Many cars use a cable to act on the engagement lever, instead
of hydraulics. A few, such as pre-war MG's, utilize a pedal
connected directly to a shaft through the transmission. A sudden
clutch failure is most often caused by hydraulic failure, (or
broken cable). Slow deterioration could be anything. How to
determine whether a problem is hydraulic or the clutch itself: Get
somebody to activate the pedal. You should be able to see the lever
on the slave cylinder move back and forth about 1 to 1 1/2". If
not, you have a hydraulic problem. If you have sufficient lever
movement, you probably have a clutch problem, and it's time to
remove the engine or transmission. The total to replace all the
parts (disk, pressure plate, throw-out bearing) may be about
$75-150 for most cars. The flywheel might need to be turned, to
eliminate warpage, scoring, or glazing, or even replaced if there
is cracking. The labour time will be higher if you have to remove
the engine to get at the clutch. On some cars, you can just remove
the transmission, and for many front wheel drive cars, accessing
the clutch is as easy as removing a cover plate or two.
At rest, fluid is free to flow from the hole below the
reservoir into the cylinder. (or visa-versa)
When you step on the pedal, it moves the push rod/piston
assembly. (To the right in diagram.) This first covers the inlet
hole, and then proceeds to push fluid through the outlet pipe, to
the slave. (Which in turn pushes a piston in the slave, and moves
the clutch arm.)
When the pedal is released, the piston returns, due to back
pressure from the clutch springs on the slave, and (in some cases)
a return spring in the master cylinder. Fluid flows back into the
master cylinder from the slave/pipe. When the piston is fully
retracted, the inlet hole is once again uncovered, and fluid may
flow in to make up for any losses. (or out, if the slave has
returned farther than it's previous position.)
POSSIBLE MASTER CYLINDER FAILURES:
Front seal fails, or pitting of cylinder wall. Fluid leaks back
towards the reservoir instead of pumping out the outlet pipe. This
will result in reduced or no slave piston movement. The effects may
vary with a borderline seal. (rebuild or replace master
cylinder)
Air in system- master, line, or slave cylinder. Air compresses,
resulting in reduced piston movement, as above, & soft pedal.
(bleed system)
Rear seal fails, fluid leaks out the back of the master
cylinder body, possibly onto the drivers foot. (R/R master
cylinder)
Piston doesn't return completely due to crud or rough surface.
Cylinder remains partially pressurized, and slave doesn't return,
leaving clutch in intermediate state. This may leak back slowly.
(R/R master cylinder)
Blockage of inlet - shortage of fluid, reduced piston travel,
mushy pedal. (R/R master cyl.)
Blockage of outlet -Hard pedal &/or fluid blows back past
front seal. (R/R master cylinder)
Clutch Slave Cylinder
++
OPERATION:
REST POSITION. NEUTRAL PRESSURE.
STEP ON PEDAL- FLUID FROM MASTER CYLINDER PRESSURE PUSHES THE
SLAVE PISTON AND PUSH ROD, (TO THE LEFT IN DIAGRAM), WHICH MOVES
THE CLUTCH ARM, WHICH PUSHES THE THROW-OUT BEARING INTO THE
PRESSURE PLATE TO DISENGAGE THE CLUTCH.
RELEASE PEDAL. FLUID PRESSURE FROM MASTER CYLINDER IS RELIEVED.
SPRING FORCE OF CLUTCH/CLUTCH ARM, (AND PISTON RETURN SPRING, IF
FITTED) PUSHES THE SLAVE PISTON BACK TO THE REST POSITION, WHICH
FORCES THE FLUID BACK INTO THE MASTER CYLINDER.
FAILURE MODES:
Seal fails, fluid blows past piston (and eventually begins to
leak out of the boot). Noticeable as fluid loss in the master
cylinder reservoir. Piston travel is reduced or eliminated. (this
can result from a torn or rotted seal, or pitted cylinder,
etc.)
Piston seizes or drags. Clutch may be engaged, disengaged, or
sluggish, depending on the position and amount of movement of the
piston. Caused by corrosion, crud, shredded seal, etc. This may
result in a hard pedal, and/or failure of the seal. This could also
destroy a weak seal in the master cylinder. (R/R slave
cylinder)
Air in system. Reduced piston travel, clutch won't fully
disengage. (bleed system)
Bad/missing lever or piston return spring. (This varies with
design).
COMMENTS:
Cylinder repair kits are cheap. Even a new cylinders can be
reasonable. Cylinders may be honed lightly (Be very careful with
alloy cylinders!), and should be polished using crocus cloth or
fine metal polishing compound.
Remember- USE CASTROL LMA GTX or other DOT4 eq. or better brake
fluid, NOT DOT3 in your British car! DOT4 should be changed every 2
years.
DOT5, Silicon is OK too. I'm currently running some long term
tests on it, with good results. It should perform much better for a
low usage or stored vehicle as it won't absorb water, and therefore
should result in very low corrosion levels. DOT4 may be preferable
for high use, or competition vehicles.
Brake systems are similar- they just have more cylinders,
tubes, and other fittings to deal with.
Pedals and Linkages
The brake, clutch, and accelerator pedals on many British sports
cars are located by a shouldered bolt or shaft passing through a
simple bushing in the pedal. These bushings are often not
lubricated, and can result in worn shafts/ bushings, and a sloppy
pedal. An even worse problem, is the linkage to the brake and
clutch master cylinder pushrods. This is often just a hole drilled
through the pedal body, with a clevis pin joining the pushrod and
pedal. Wear at this joint is often excessive, resulting in an
ovaled hole in the pedal body, and a narrowed clevis. Due to the
mechanical ratios, a small amount of wear at this joint will result
in a large reduction in pedal travel. You can check this some by
wiggling the pedal up/down, and side to side, but visual inspection
is best. The fix is simple.
First, remove the pedal assemblies from
the car. Fill in the ovaled hole by welding or brazing, and
re-drill the hole. This should be a fairly close fit, probably no
more than .005" of slop. Ream to fit if necessary. If the push-rod
holes are worn, they may be repairable as well, or may be replaced.
If the pivot bushings or shafts are worn as well, now's your chance
to fix them too. When reassembling, coat the pivot bushings with
some good wheel bearing grease. Fit new clevis pins, which only
cost about 50 cents. Do not skimp and use a threaded bolt instead,
as both it and the pedal will wear away quickly. A couple drops of
ATF, or possibly motor oil here completes the job.
Source:
________
/___ _ \ Roger Garnett (Roger_Garnett@cornell.edu)
/| || \ \ Agricultural Economics | South Lansing Centre For
| |___|| _ | 3 Warren Hall | Wayward Sports Cars
| | \ | | | Cornell University | (Lansing, NY)
\| \ |__/ / Ithaca, N.Y. 14853 | (607) 533-7735
\________/ (607) 255-2522 | Safety Fast!
Copyright ? 1992-1995 Roger Garnett You may publish this in
your club newsletter, provided full credits are given, and you send
me a copy.
Copyright held by origional author. Used according to author's
specifictions.
OVLR, Inc. web page
Last modified 30 October 1995. Comments
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