Mystery Clutch Problem (clutch slowly rises to the up position)

[Nfsundrgrnd]

I have a 2006 touring edition 350z and I've been trying to fix a mystery clutch problem for the past 2 months and have not been able to fix it or find the cause after spending over 2000$

 

The issue started when I was in traffic and the clutch was working perfectly fine when all of a sudden, after letting off the clutch to inch forward, it did not spring back up... It slowly came back up after roughly 3 seconds. My Initial thoughts were that my master cylinder let go or that I blew a clutch line. 

 

After bringing my car to the mechanic, I was told that it was most likely my throwout bearing, so I told him to replace the entire clutch kit while he was in there. After getting an Exedy stage 1 clutch kit put in (I have a JWT lightweight Flywheel from the previous Clutch kit), the exact same issue was still there so I told him to replace the master. (the slave had been replaced the previous year) after all this was replaced, the exact same issue was still there (clutch slowly rose back to the up position).

 

At this point, it was assumed that there was still air in the system and so the clutch was bled several times over a week. While this improved the problem slightly, the problem would be the exact same once the fluid warmed up. The mechanic then replaced the slave just to be certain that it was not the issue, and surely enough it made no difference. 

 

There is no visible leak anywhere and the reservoir is still full. The only thing that was not changed was the clutch line and I'm currently in the process of sourcing another one. The pedal also seems to stiffen up when it is pressed rapidly and the other day, had completely locked in place after attempting to shift quickly from 1st to 2nd. After giving it a few seconds and pressing the pedal slowly it was working normally again (still with the same issues though). To give you an example of how badly it locked up, when I attempted to bleed the clutch myself the following day, there was fluid leaking out of the push rod seal of the master (possibly blew the seal), but I was not able to get it to leak again so that is also a mystery. 

 

Now I am at a complete loss and have no idea what it could be as we've replaced nearly every possible part that could be the cause.

 

I'm hoping that you can shed some light on this issue and possibly help me find a solution.

 

Thanks.

[mr v6]

The usual reply here, I've been there, is to replace the standard line with a braided steel one & thoroughly bleed the system till you're bored of doing it. I also added a length of exhaust heat wrap around the line. This was 2 years ago 7 the pedal feel since has been spot on. Another culprit is the clutch slave cylinder, whether yours is the external or internal type.