When I had this happen to me I replaced everything from the master cylinder, the hose, the slave cylinder, the piston, the clevis pin... and lastly... the arm.  Turns out it was the arm that was the problem. The little ball had cracked and slowly cracked until it eventually fell off. If I had checked that first I could have saved a lot of time and money.   Have you checked the clutch arm yet?

Sorry all, the shim behind the bearing is certainly only to make sure the bearing turns freely. When I started the clutch journey with my Mini I was told by others that it would make the clutch release point earlier giving more clutch pedal travel. It doesn't. I was focused on that when I wrote the long story above.

Just to be clear, the shim (or gap) between the TO bearing and the shoulder on the plunger is to allow the TO bearing to spin freely. The race on the "heavy duty" type bearing comes in contact with the shoulder (or seat, or step, etc.) and doesn't allow the bearing to spin. The gap or shim is not used to adjust the clutch in any way.

Ha! I should have known this! I rebuilt a combo clutch/brake MC for an MGA years ago and managed to put the slow-fill valve on the clutch side. After running through gears a couple of times the clutch begin to slip. When I gave it a rest, the slipping stopped – and then started when I began using the clutch again. A real head scratcher until someone suggested I may have installed the slow-fill valve incorrectly.

And yet, I swear it helped to pump the clutch on my Mini. Of course, it did not. BTW: I replaced the clutch MC a couple of days ago and all seems well.

Pumping a clutch pedal (according to the manufacturers) doesn't do anything but make you feel better !

As there's no slow-fill valve in a clutch m/c, pumping the pedal quickly physically can't build build up pressure - I know we all do it and swear it makes a difference, but it shouldn't...

Brakes are different, and the m/c is designed to allow for safety pumping.

I would have preferred to install the TO bearing shim now that I know the outer race of the bearing interferes with the shoulder of the plunger. But the machine shop that pressed the bearing on for me said, "That bearing's not going anywhere, the press fit was very tight." I guess we'll see. But my current problem is air getting into the clutch line. Pumping the clutch peddle and/or bleeding the slave cylinder temporally solves the problem. My slave cylinder is about a year old, so I'm hoping a new MC does the trick. I'll find out soon enough.

So, if you left a gap between the throwout bearing and the face of the plunger, every time you push the clutch pedal the bearing will tend to be pressed onto the plunger to eliminate the gap you left. There is no adjustment of the clutch system other than the release stop that will improve operation. The only way to make sure you have the maximum effective clutch pedal travel is set the system up from the diaphragm spring to the pedal. Any shim added between the throwout bearing and the plunger will not change how the clutch releases. So, all new master cylinder, pins, clutch cylinder, throwout arm, plunger, diaphragm spring and properly machined flywheel and pressure ring set will give you the best clutch action you can get. But only if you shim the flywheel, pressure ring and diaphragm spring properly.

Let the yelling at me begin now as this is always a "Great Discussion" among Mini owners. I'd include a roll eyes emogi, if I could.

I'll have a look at the slave cylinder when I replace the MC. It's possible that I'm having the same trouble with snap ring and plunger.

Not sure I follow your comments re: arm and release bearing. On the other hand, I reciecently replaced my release bearing w/ the heavy-duty version. The bearing came with a washer (shim). I called the supplier and he said to toss the washer as it was not needed. When it came time to press the bearing onto the plunger, the bearing bottomed out and refused to turn. As I had already tossed the washer, I simply backed off on the bearing and left a few mil space to allow the bearing to turn. That seemed to do the trick. But I'm still having trouble w/ air in the clutch hydraulics. Hope the new MC solves that. 

Check the stroke on the slave. I had one car that when pressed the clutch the slave cylinder piston was hitting the snap ring and then would drag stuff and air back into system. Once realized what was happening changed arm and shimmed the plunger behind release bearing as the new release bearing was smaller than original and did not notice previously. Once done no more issues. May help
bruce

Did you peel back the dust boot on the slave cylinder to check for leakage? With the quality of parts available these days, recent replacement of a part doesn't remove it from the list of suspects. How about under the master cylinder inside the car?  Is it dry?

It seems like everything is pointing to the the MC as being the culprit. But it's weird that it would suck air, but not leak fluid. Re: DOT-5 fluid. I considered running DOT-5. I like the fact that it doesn't absorb water or damage paint. But after learning I'd have to drain and flush out every bit of the existing DOT-4, I changed my mind.

I presume you adjust the return nut periodically ???  

Try running without the return spring for a while...

Cheers, Ian

Time for master cylinder rebuild or a new one. Try DOT 5 in the clutch and brake systems.

Over the past couple of months my clutch pedal has required more and more travel before it disengages the clutch. Pumping the clutch periodicly seemed to help. Finally, two weeks ago the pedal went all the way to the floor w/o disengaging the clutch. No amount of pumping would help. I bled the slave cylinder and that solved the problem and brought the clutch pedal back to normal. 

A couple of days ago the pedal travel issue was back, and today the pedal went all the way to the floor again without disingaging the clutch. The slave cylinder and my clutch arm, clivis pins, etc. were replaced about a year ago. Am I correct in thinking the problem is my clutch master cylinder? Everything looks dry and clean in and around both cylinders and I'm not losing any fluid.