Originally Posted by Fountainhead

My personal theory is that high rpm shifting/hard launches cause the thrust bearing/throwout bearing to heat up immensely, and then this of course heats up the entire CSC assembly, which then cooks the O Ring and then bam, you no longer have a seal. Clutch nasties in the fluid don't help either.

The stock Z is a "Sports" car, not a "Race/Drag" car, and it just can't handle rough treatment without modding money spent on weak areas.
I'm not trying to justify the plastic CSC, it should be more robust, but if that didn't break, something else would from the rough treatment.

What causes the Master Cylinder to fail? It's cheap I guess, and can't withstand repeated scraping of the O Ring in oil with clutch fluid nasties in it.