Is something wrong with my Clutch Master Cylinder?
 

I need some advice from other people who have bled their clutch using the manual method (open bleeder, press clutch pedal, close bleeder, pull clutch pedal). I connect a 1/4" clear plastic tube to the bleeder valve to catch the outgoing fluid and to observe any air bubbles. For the longest time, I've been bedeviled by these tiny air bubbles that look like champagne bubbles. I've gone through a whole jumbo sized bottle of brake fluid, and these bubbles still keep coming. I recently tried a new technique. I tried pulling the clutch pedal up *VERRRY SLOWWLY* every time I closed the bleeder valve, taking about 2 seconds to complete the pull. Then all of the champagne bubbles disappeared! It really seems to me that somehow, the clutch master cylinder is introducing air when I pull it up too fast. In all of the how-to posts that I've found on the internet, nobody ever said you had to pull the pedal up slowly, so I am wondering if there is a defect in my CMC? If you also do the manual bleed method, do you pull the pedal up slowly too? Also should mention that I keep the brake fluid filled to the top of the fill neck, so there really isn't any possibility for air to get in through the reservoir. Also should mention that in normal operation, there is no fluid leak anywhere, and the fluid level never drops in the reservoir. Therefore if there is a path for air to get in, it would have to be in a unpressurized section of the clutch fluid line, ie in the CMC body behind the piston.

The clutch engagement point is extremely low when the car is cold, and rises noticeably after driving for a few minutes. This behavior seems to be minimized immediately after doing a thorough bleed, then starts creeping back in. And yes, I used teflon plumber's thread tape on the slave bleed valve. My question to anyone who has experience is, is it normal for air to be introduced to the clutch line when you pull up too fast on the clutch pedal during bleeding?

If the bleeder is closed the answer is no.

The last time I changed the slave and associated line on a Mitsubishi (a few years ago) I had my wife depress the clutch pedal once. Some air spewed out followed by clear fluid. I closed the bleeder and done.

How could you be sure that there was no more air in the line after just one press of the clutch pedal? There could have been a bubble at or near or inside the master cylinder.

A couple of issues you could be having:

The bleed line you connected to the slave could be pulling in air try using a zip tie to make sure it seals tightly around the bleed valve.
Old CMC slave cylinders have sediment and crap that builds up over time. This causes it to be difficult to bleed unless they are cleaned out.

Try the following:

The CMC is adjustable inside the car. You can make the engagement point closer to where you want it.
You could also try to gravity bleed the CMC. That just takes time and a steady supply of fluid.

Thanks for your reply. Because you are suggesting other possible causes for the air bubbles, I infer that you have done the manual bleed method described in my original post, and that you pull the clutch pedal back QUICKLY on the upstroke, and have no problems with air getting in. Is this an accurate inference?

Its just a line from the master cylinder to the slave, no abs or proportioning valve to harbor air. Clear fluid was coming out, the clutch worked fine afterwards.

But my point is that one press of the clutch pedal is not enough to displace all of the fluid in the line. If there was an air bubble in the fluid next to the master cylinder, that bubble will definitely not make it out of the line in just one pedal press. How you perceive things being "fine" could be very subjective and could also depend on the size of the air bubble, if there are any.

The point is, the slave is very simple to bleed out. The fact that you keep seeing air means something is wrong.

If I was having your problem, and I knew I wasn't just running the reservoir dry, I'd pop for the master and slave and maybe the lines in between and then move on with life.

Thanks for that advice, but I don't agree with that approach to car repair and maintenance. I'd rather figure out what's wrong instead of just replacing everything willy-nilly (expensive!).

Huh.. I can ask my friend about this...