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I found the below posts on a different saab group and thought it was important to be included here. Conclusions to be drawn IMO, the most likely/common failure mode for the oem cylinders is......
Slave Cylinders - worn or improper seal/oring and decaying hose bits in fluid
Clutch Master Cylinder - flexing of piston causing axial scoring of bore
DanaH
97 9000CS, 160k miles with orig master cyl and 2nd slave
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Why 9000 Clutch Master Cylinders Fail & Mods.
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After having a number of newly purchased "OEM" clutch master cylinders fail after only a 2-3 years in service compared to about 10 for the OEM Saab cylinder, this had to be explored.
Taking apart three of the failed clutch masters revealed that the failures are due to axial scoring on the ID of the cylinder's bore causing the seals to leak. It appears the piston is rocking as it travels the distance of the cylinder bore under pressure and this rocking causes the piston guides to scrap axial lines on the cylinder bore.
The piston to bore clearance on these failed cylinders are about 0.003" and if all the seals are removed, the piston rattles inside to bore.
The old OEM Saab cylinder has much tighter clearance, about 0.001" to 0.002" at locations were there was signs of wear.
What appears to have happened is the current clutch masters run a loser tolerance resulting in shorter life.
With this in mind, a high precision SAE 841 bronze sleeve was made to fit near the front of the piston to help guide the piston under pressure and prevent it from rocking inside the cylinder bore as it moves.
Using a "OEM" after market clutch master, it was taken apart and measured using an internal bore micrometer and the piston guides measured with a micrometer. These measurements are accurate to better than 0.0001".
The measured bore ID = 0.7015" after honing with a 200 grit flex hone.
The Piston guide area OD = 0.6970" to 0.6940"
Dimensions of the bronze sleeve:
OD = 0.6995"
ID = 0.3830"
Wide = 0.340"
Completed sleeve was cut in half using a 0.0400" slitting saw.
The sleeve is precision fitted the gap between the front piston guides.
The modified piston was test fitted into the cylinder's bore.
The rattle is gone with a nice precision fit and feel.
New seals installed, cylinder and seals lubricated with Krytox O-ring lube and assembled.
Next step is to fit this modified clutch master cylinder into the 93' Aero for testing. At time, all new hoses will be fitted.
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Only time will tell if this rebuild from an aftermarket (does appear this could have been an OEM cylinder as 'SAAB' stamping was ground off, or this could have been a repackaged reject). It was in service for about three years before this cylinder started to act wierd... low pedal and spongy pedal feel.
At that time it was removed and another one was installed.
This particular cylinder does have some axial scoring, that honed partly away, but not completely. It does appear surface defects on the cylinder walls are to cause of failures in these cylinders, not seal problems.
The sleeve was made in the garage shop, we have a lathe and mill and...
There are no plans to make more as the problem lies in tolerances involved and this fix has not been tested enough to prove it properly corrects this problem.
All three units inspected have different bore diameters ranging from about 0.7000" to 0.7020" and the pistons all have different guide diameters and the ID of the sleeve is fitted specifically to the specific piston assembly. Each sleeve must be made specifically for each cylinder.
All of which points to the core problem of production tolerances with these clutch master cylinders.. they vary significantly from unit to unit today.
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I was welding up the 1/2" crack in my clutch pedal mount bracket a couple days ago and realize the following. Even though it's now welded up and stronger than before the bracket still flexes with every stroke of the pedal.
This gives the potential to lateraly load (stress ) the piston with every stroke. This would/could severely shorten MC life...
I don't know if there's another crack on the back side of the mount that I can't see while the mounts in place (I welded insitu) or if the design is simpley too weak for long life... I've looked at other mounts in the junkyard that didn't appear to be cracked and the mc still flexed so I'm inclined to think the original design is lacking.
I'll post up with pix when I design a better reinforcement for the bracket.
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'That crack on the pedal bracket cannot help the problem. It is likely due to a sharp edge stress riser that was created during metal stamping and not smoothed off. After many, many, many cycles of flexing, that stress riser grew into a crack that grew even further and will continue to grow. The apex of that crack needs to be carefully drilled out to a small hole (2mm is fine), all the sharp edges smoothed out then it can be welded up and reinforced to stiffen. Any repair less than this will result in another failure at some point in the future.
Decided to replace all the clutch hoses in the Aero. During that process, decided to replace the clutch master cylinder. After all was done, the new NAPA/Perfection clutch master with "Saab" ground off and the correct part number stamped on failed after no more than 100 pedal strokes or 5 test miles... This one would not hold pressure and the piston would stick at the bottom of it's travel. Clearly defective "new" clutch master cylinder. Took it apart and measured the piston to cylinder clearance, which turned out to be about 0.004" and there was already axial scoring marks on the piston. I'll try to get some images of this defective master up later.
Put the original one back in resulting in a working clutch again...
I have contact with one of the west coast reps/importers for Pro Parts Sweden and discussed this problem with him. I'm going to try and get them to make a much higher quality clutch master cylinder as it appears the current replacements are mostly junk.
BTW, the same guy who started Scan Tech sold the company to Dorman Industries which went on to produce mostly junk after market parts for Saab. Some time later after Scan Tech died, the same guy started up Pro Parts Sweden and it appears they are trying to make quality after market parts for Saab again.
posted by 170.63.74...
, Fri, 7 Feb 2014 15:25:57
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