Just took a stab at some of this theory. I'll post my thoughts and have another read through here and elsewhere. I'm feeling my clutch is probably just toasted at this point.

Wanting to rule out the pressure build up theory at the CMC, I adjusted the CMC rod at the pedal to allow full return of the master piston. After doing so, thethe clutch behaved the same and slipped in the same situations. The only difference being more requires pedal depression to achieve release.

I checked the CSC to assure there wasn't any force being applied against the fork and that the fork had the room required to allow a full release. I was able to wiggle the fork at the slave rod with minor effort as well as push the slave piston inward.

This leaves two possibilities (I think)

1. The clutch is just toast.

2. The collar is actually just tall enough to always have some pressure against the pressure plate fingers, when it's bottomed out. (Unlikely)

From L to R. My collection of collars. '70 4 speed B, '73 4 speed A, '79 5 speed B.

Just took a stab at some of this theory. I'll post my thoughts and have another read through here and elsewhere. I'm feeling my clutch is probably just toasted at this point.

Wanting to rule out the pressure build up theory at the CMC, I adjusted the CMC rod at the pedal to allow full return of the master piston. After doing so, thethe clutch behaved the same and slipped in the same situations. The only difference being more requires pedal depression to achieve release.

I checked the CSC to assure there wasn't any force being applied against the fork and that the fork had the room required to allow a full release. I was able to wiggle the fork at the slave rod with minor effort as well as push the slave piston inward.

This leaves two possibilities (I think)

1. The clutch is just toast.

2. The collar is actually just tall enough to always have some pressure against the pressure plate fingers, when it's bottomed out. (Unlikely)

From L to R. My collection of collars. '70 4 speed B, '73 4 speed A, '79 5 speed B.IMG_20150729_170917.jpg

 

It sounds like you simply need a new clutch. If the collar was too long you would not be able to move the release arm by hand because there would always be pressure on it from the pressure palte and the slave would be bottomed out.

I'm thinking so too. Just threw in a shorter collar for giggles... so far it passes the jackstands test. Going to try a short drive. There is lots of crap on the inside of the bellhousing. Maybe this clutch is just oil contaminated, or its just the cheap ebay clutch thing. I have about 2500 abusive km on it.

Interestingly enough. Test drive allowed wheel spin and clutch is behaving more or less as it 'should' using the shorter '70 bearing collar.

I'll follow up if things change.

Here's another - http://www.classiczcars.com/topic/44389-l28-is-toasting-my-240z-clutch/page-2

 

92-95 mm from the fork surface of the collar to the flywheel surface is the key.  That puts the end of the fork in the right spot for the slave cylinder and its rod.

Did you measure the height of the assembled parts.  Can't emphasize enough that this will tell the whole story.  The slave cylinder and the clutch fork pivot ball have the same relationship on all transmissions, from 71 to 83 for sure, maybe even earlier, because they're both mounted on the transmission case.  It's that relationship that determines the fork angle after the transmission is mounted to the engine, and how far the collar and bearing move when the clutch pedal is pressed.

 

I will never install another transmission without verifying that measurement.  

I literally reached inside, swapped collars and lifted it back into place. Once I remounted the slave, I just spun the adjustment nut until it took firmed up against the fork.

Next time it's out I'll measure everything. This was kind of a quick and dirty experiment I didn't expect to achieve anything with. Figured I'd be taking measurements and placing an order for a clutch tonight.

Is it 92-95 mm with the TOB resting on the fingers without pressure applied?

I wouldn't think a clutch would be toast after 2500 km unless you were doing high rev launches over and over. That's like 1,500 miles even a sorry clutch aught to hold up longer than that. I think you are looking in the right area from the sounds of it.

3 hours of hooning last night says clutch is holding up fine. It was the collar.

Very cool. Glad you fixed it.