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M90 slave cylinder failure or overtraveling?

redblockpowered

bad username length
Joined
May 5, 2015
Location
Swampscott, MA
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?
 
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.
 
I found out that the slave cylinders have different internals. A fix is the 5mm trick for the pivot or 20mm pushrod extension
 
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.
 
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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.
 
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.



clutchwood.jpg
 
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.
 
Ahh, the joy of bathing in the filth of the brake fluid...

Found this thread in a Google search. I've also had the slave cylinder piston and seal over-travel and pop out of the housing after my Sachs -763 pressure plate pushed the rod straight through the hole in the clutch arm; what an annoyance! I welded on a washer near the end of the rod to prevent it from going through. The pivot socket in the middle of the arm was also worn thin and cracked, welded that too. I guess I was supposed to keep it greased/lubricated, but I didn't.

Keep in mind that many of these old Volvos can have three different slave cylinder ID's to choose from: 19, 20.5 and 22.2 mm, depending on transmission and model year, M90-type is supposed to use the largest one only. One parts retailer here in Norway even lists a different master cylinder part number for 940 station wagons VS. sedans; who even knows what that's about? From my investigation the 940 master cylinders are all supposed to be 19 mm ID.

sbabbs idea of a longer rod between the slave cylinder piston and clutch arm is a good one; the piston appears to have plenty of travel, but not nearly all of it is being used and there's a still a lot of travel before the piston will bottom out in the cylinder.

EDIT:
KL Racing's website lists a clutch master cylinder 19 mm ID, not for M90's and a seal kit, clutch master cylinder, 19 mm ID, not for turbochaged gasoline engines, implying that the turbo models use a different type of master cylinder? I have a '91 non-turbo project car (dead M47-II #2) and a '96 low pressure turbo (good M90) parts car. I've quickly compared to the master cylinders of the two and they appear similar, but not identical. The jury's still out as I still need to verify the number cast on the bottom side. Both measured 28 mm OD in the vertical plane.
 
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What kind of pivot bolt is in there? It's been a long time since i installed an m90 on my 780 but i know i shimmed the bolt out using a few washers.
If that bolt comes forward you might not need a longer rod.
Messing around with the rod lenght is no fun. We did it once on a rally car that got a Selholm twin clutch. I think we had it in and out 4 or 5 times before the length made the slave cylinder work acceptably.
 
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