The fundamental feature of the clutch hydraulic system is that there is an
unobstructed 'up hill' path for air from the slave cylinder through
to the reservoir.
A bubble sneaking past a worn slave cylinder
seal will rise to the top of the slave cylinder, into the pipe and
up to the bottom of the master cylinder, into the master cylinder
and upwards to the pressure equalisation hole just ahead of the
front seal, and finally up to the reservoir.
Once the system is partially 'bled' and you have some pedal resistance, the system
is essentially self bleeding. By the way, it is common for old
slave cylinder seals to admit a little air into the clutch
hydraulics and the self bleeding action usually gets rid of it. Some
of us have developed a soft clutch after extended periods of highway
driving, which seems to cure itself. This is an example of the
air leaking in faster that the self-bleeding can get rid of it.
Given the above description you can see why reverse bleeding is
recommended in the manuals. However there are better and
easier ways to get the job done: With the bolt on type of slave
cylinder you simply unbolt it, let it hang down and open the bleed
nipple. If the system is OK then the cylinder will fill with fluid
and eventually pour out of the bleed nipple. At this point close the
nipple and refit the cylinder. Doing this will necessitate pushing
the slave cylinder piston in, displacing fluid up the pipe and into
the master cylinder.
Now the system is partially 'bled' and
you will have some pedal. Start the engine to provide some
vibration to facilitate the movement of bubbles, press the clutch
pedal to the floor, wait 5 seconds, release the clutch pedal and
repeat until a full pedal is obtained. With the clip on type of
slave cylinder you achieve the initial bleeding by releasing the
clips that locate the cylinder, turning it up side down (bleed
nipple pointing upwards) and allowing it to move forward. Open the
bleed nipple and when fluid flows out of the nipple, close it and
move the cylinder back to its normal position (with bleed nipple
down). Complete the bleeding as outlined above.
What can go wrong with this procedure? Essentially anything that
prevents fluid flowing from the reservoir down to the slave
cylinder; a blocked pressure equalisation hole in the master
cylinder or a flexible hose which is internally obstructed. The best
test for this is to open the bleed nipple at the beginning and make
sure that fluid flows out freely. The other potential problem is a
worn slave cylinder seal that allows too much air into the system as
you are trying to bleed it out.
Another thing to watch out for when there does not seem to be enough clutch travel to properly
disengage, is a worn pedal to master cylinder push rod. These
rods wear where they go through the pedal and sometimes wear through
and leave you stranded with no clutch.