Padhammer

I have two 968s. Both had the wrong (993/964 Heavy Pressure Plate). I am using the true green strip Porsche PP (not Sachs 993/964 sold as a 968). I changed everything except the DMF on one of them about a month ago and everything was fine. Now clutch is soft butter smooth - love it.
I changed the PP, Throwout, and needles on the other one yesterday. I reused the fork pin and the guide - both looked OK (I used high pressure grease per Porsche parts number on appropriate locations).
But, the reservoir was at the min when I started (my bad) and when releasing the slave I must have sucked in air. So, I used a Motive to bleed - allot of air bubbled out into the catch bottle. Now the clutch pedal works fine sitting still until you put into gear and move the car - about 8 out of 10 gear shifts the pedal sticks at the bottom or half way up - have to pull it back by hand and it pivot snaps like a vise grip. When it is stuck, the mechanical clutch is engaged because I cannot put it in gear - so even though the pedal is down, the clutch is not in the released position.. If I pull it back up, it works fine again as long as sitting still. But same pattern repeats once engaging gears and moving a bit.
So, I re-bled the system. I removed the slave and tilted the back end upward. No air bubbles seen. I jacked the rear of the car up high - this should allow air in the master to escape out the blue hose. I re-bled and no air bubbles. I have a very hard time believing here is still air in the system.
When the slave was off, I checked the fork for free movement - everything seems fine. I payed on the fork and the PP appears to function in and out correctly.
I used both shims on the throwout - could still push the throwout around but it was stiffer than on the first car where I believe I only use one shim. Is it plausible that the throwout shims have it too tight and it is sticking to to the guide tube?
Or is there any other way I could have another mechanical (non-hydrailuic) issue?
I checked the firewall and don't see any cracks at master.
I have a hard time believing the master just happened to fail at this time unless air getting in it could have somehow damaged it?

I'm at my wits end. I'd really appreciate any help anyone can provide. There must be a major clue here in that it only happens when actually shifting gears - otherwise its fine.

Thanks in advance for your time - it will be GREATLY appreciated.

DSMblue

Hmm, if the master failed, you would most likely see fluid leaking out into the driver footwell. If there isn't any leakage, you can probably rule out the master. Beyond that, I'm kind of at a loss.

Padhammer

No leaks into the car that I could see. I got up under there to confirm the 147mm and 65mm lengths and everything was dry where master shaft comes through the fire wall unless its all seeping behind the insulation. But, as many times as I pressure bled I would imagine that I'd be smelling fluid in the car by now.

Zirconocene

Before you take things apart (not that it sound like you were ready to do that) have you tried to reverse bleed the system?

The other thing that occurs to me is: have you checked the blue hose? Any indication that it's leaking?

ETA: Have you taken a look at the spring on the clutch pedal itself? Perhaps a little judicious lubrication on those parts might help.

Cheers

Padhammer

I haven't tried reverse bleeding. I think that is a good idea - I have a large syringe but its outlet is too small for the bleeder connector tubing. Does anyone have a "parts list" for this job.

Yes. blue hose is in good shape.

I only know of one spring in the clutch pedal assembly and it doesn't appear to be an item the should require lubrication, but I'll take a look.

Yes, I've written this post is hopes of avoiding just randomly removing parts.

My biggest quandary is why does the clutch work fine until it is rotated? Then after one sticking pedal release, you can work the clutch all day long and its fine until its rotated again. I would be really interested to know if anyone that has experienced air in their pedal hydraulics has had a similar experience. In all other cars I've worked on, the clutch acts the same way when air is in the system regardless of whether sitting still or attempting to drive.

jsheiry

On the 924/944/951/968 Tech Forum of this website this video was shared by 931Guru which is very interesting take on the system. Worth a try.

Padhammer

Update:
I have bled the system multiple times using multiple methods: Reverse Bleeding, Motive pressure bleeding, Rear of car jacked up high and bled, bled with clutch pushed to the floor first, removed slave , tilted it up and pressure bled it.
No indication of any remaining air
No leaks detected
Clutch works fine , over and over again sitting still with or without engine running.
I've performed the following test at least 10 times:
Start it, put it in gear, release clutch slightly, car moves (clutch engaged), clutch pedal is stuck to the floor - turn engine off, pull pedal up, test pedal (it works fine again), repeat moving test and same thing happens.
Just as a sanity check that clutch is working, I started car in gear with the clutch depressed and it started up fine (starter was not driving the car). When I released the pedal , clutch engaged but pedal stayed stuck at the floor.

Am I correct in my understanding that the pedal should act the same with or without the car running. In other words, the successful repeated cycling while parked (with or without engine running) "proves" ther is no air in the system?

I am completely baffled. Before I go tearing things apart, I'm open to any suggestions / ideas on what could be happening.

Thanks in advance.

Jfrahm

Have you tried forcing the slave piston back in? Maybe with a speedbleeder in place that would get it.
The other tricks I have used to great effect mostly on motorcycles:

Bleed at the other fittings in the system, under power bleeder pressure. If that does not work:

Maintain pressure via the pedal overnight or for a an hour or two, then crack a fitting or bleeder. Repeat for the other possible locations. The pedal pressure, far higher than a power bleeder, makes the bubbles very tiny and the fluid moves fast and far when released.

If you get it working but the pedal feels weird, just drive it (with some abandon) and the last of the air will find its way out.

Cheers, -Joel.