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Old 05-27-2019, 03:10 PM   #1
redblockpowered
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Default M90 slave cylinder failure or overtraveling?

I just finished putting an M90 in my 940. I used a dog dish flywheel instead of the stock dual mass unit, with 940 diesel clutch setup and turbo pressure plate. I shimmed the pivot ball 3.25 mm (measured with a digital caliper). Around 200 miles afterward I suffered a loss of clutch pedal pressure after holding the pedal down for 15-30 seconds. Looking underneath revealed that the slave boot had blown off and pissed fluid all over the ground. Obviously this means the slave cylinder is allowing fluid through. This morning I took the slave off (which I had given a new seal installed in the correct direction before mounting) and found... nothing of note. The seal looked fine, the bore was clean and without scoring. Did I not shim the ball enough, allowing for the slave to overtravel, or do I simply have a failed slave cylinder that looks and feels normal?
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Old 05-27-2019, 03:27 PM   #2
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I think the piston has come so far out that the seal is outside of the cylinder.
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Old 05-27-2019, 08:05 PM   #3
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Longer slave push rod to the fork is what I do. I take a longer bolt and grind the ends so there round.
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Old 05-28-2019, 01:53 AM   #4
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Quote:
Originally Posted by Lankku View Post
I think the piston has come so far out that the seal is outside of the cylinder.
maybe it came out due to installing a new clutch, is now back into position and left you with a rubbercollar full of brakefluid.. i once had that...
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Old 05-28-2019, 04:21 AM   #5
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Originally Posted by MadDog_945 View Post
maybe it came out due to installing a new clutch, is now back into position and left you with a rubbercollar full of brakefluid.. i once had that...
It's a possibility.
And now that I dig my memories, there have been 2-3 cars with a new stock clutch and the rod had to be extended because the piston came out.
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Old 05-28-2019, 06:23 AM   #6
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The pivot needs to be spaced about 6mm to work properly in my experience.
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Old 05-28-2019, 10:37 AM   #7
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I found out that the slave cylinders have different internals. A fix is the 5mm trick for the pivot or 20mm pushrod extension
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Old 05-28-2019, 12:54 PM   #8
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Does anybody know what the proper stroke length is for the slave cylinder pushrod? We had it measured at 19mm, at which point the head of the slave cylinder piston is 6mm deep in the cylinder, and the clutch worked correctly. That put it 2mm below the bevel that's at the end of the cylinder, I assume to aid assembly at the factory. Point is that the piston is well in the bore, at least at this stroke length.

Last edited by shoestring; 05-28-2019 at 01:01 PM..
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Old 06-04-2019, 10:47 PM   #9
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Changed slave cylinder AND bumped the shim up to 1/4". Scientific? No. Will it solve the problem? Yes.
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Old 06-05-2019, 09:24 AM   #10
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Quote:
Originally Posted by redblockpowered View Post
Changed slave cylinder AND bumped the shim up to 1/4". Scientific? No. Will it solve the problem? Yes.
Youthful optimism.
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Old 06-05-2019, 12:40 PM   #11
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If the piston comes too far out of the slave it can tip and jam (not return).

If the piston is too far down the slave (relaxed) it might bottom out as the clutch wears, but that is an easy fix to adjust the push rod length.
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Old 06-05-2019, 01:06 PM   #12
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I've had the clutch fork go too far and bind with the throw out bearing and stick the clutch in at the drag races no less. This happened after I made my shaft longer. How did I fix that? I zip tied a piece of wood behind the clutch pedal so I then couldn't push it too far. So keep that in mind, long shafters.



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Old 06-06-2019, 02:49 PM   #13
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omg.. this fix is more tbstyle than everything i ever saw! :D
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Old 06-06-2019, 08:39 PM   #14
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...Yikes.
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Old 06-13-2019, 05:07 PM   #15
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Ran 12.61 with it yikey bro's..
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Old 06-16-2019, 08:47 PM   #16
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My clutch fork that came from Europe has a Kevlar pivot ball bushing in it. I think without it the 3mm spacer is more like 6 or it'll hyperextend the slave.
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Old 06-17-2019, 09:36 AM   #17
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~500 miles in after the update posted above. The 6mm spacer puts the release point in a far more normal place and there's no evidence of leakage.
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Old 06-17-2019, 09:37 AM   #18
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1/4" spacer with 1/4" hole, drilled to 5/16", purchased from local Ace Hardware, provided the correct result. Piston stroke is approx. 18mm.
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