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Discussion Starter #1
After everyone left the tech session today I disassembled the slave cylinder that slave cylinder that I had replaced in my bike.

This is an incredibly simple part and now I understand why they all leak.

I have attached a couple of pictures. The first shows the components of the slave cylinder. The second picture is the piston in the cylinder and the third picture is a picture of the rubber seal. If we could find a replacement seal for this we could simply rebuild the slave cylinders and save $100 on a new one that is likely to leake also. :(

We must have a hydraulic expert out there somewhere that could spec. out a replacement, hopefully better, seal.

If all else fails maybe we could get new seals from Magura directly.

Kevin
 

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Having rebuilt several automotive master cylinders and calipers, I'm not sure what you're seeing that's a design weakness. It looks pretty similar to most any I've seen elsewhere.

If we didn't have so many leaking - I sure wouldn't be expect problems just by look'n at these pictures. What're you seeing that I'm missing?
 

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Discussion Starter #3
The seal itself looks like a pretty poor fit but I'm no expert on hydraulic seals.

Kevin
 

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BTDT I dismantled two and found that the seal sheds little balls of material as it deteriorates. It doesn't take much of a build up of this shedding to create a space for the leak. I have a good friend who designs hydraulic systems and he found a replacement that he thought would do better in this environment, brake fluid. I can't find his email right now but have asked him to send me the info again and I will pass it on when I get it.
 

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kevincook said:
After everyone left the tech session today I disassembled the slave cylinder that slave cylinder that I had replaced in my bike.

This is an incredibly simple part and now I understand why they all leak.

I have attached a couple of pictures. The first shows the components of the slave cylinder. The second picture is the piston in the cylinder and the third picture is a picture of the rubber seal. If we could find a replacement seal for this we could simply rebuild the slave cylinders and save $100 on a new one that is likely to leake also. :(

We must have a hydraulic expert out there somewhere that could spec. out a replacement, hopefully better, seal.


If all else fails maybe we could get new seals from Magura directly.

Kevin
There is one thing to think about before embarking on slave cylinder rebuild. There is a very small ball thrust bearing in the end of the piston, which is the cluth "throw-out" bearing. If this gets even a little tight, the piston will be spun in the cylinder bore, and almost immediate failure results. That is what happened to one of mine. See picture. You can see how the hard anodize coating was destroyed by the spinning of the piston in the cylinder, which destroys the seal right away. I would not want to rebuild a cylinder with very many miles on it by replacing the seal, then the next failure is likely to be the bearing. I would imagine quite a few of the really bad failures may be from the piston spinning in the cylinder.
 

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Rebuild or Replace

Rebuilding would be pretty easy.

Quote: "BTDT I dismantled two and found that the seal sheds little balls of material as it deteriorates. It doesn't take much of a build up of this shedding to create a space for the leak."
This is a very interesting observation, Jeff, thank you for this information.

Quote: "I have a good friend who designs hydraulic systems and he found a replacement that he thought would do better in this environment, brake fluid. I can't find his email right now but have asked him to send me the info again and I will pass it on when I get it."
Jeff,If you do get a substitute part number, please share it with us here.
Now I am thinking that just drilling a weep hole to let leaking fluid out isn't the only thing to do. Proactive change out of the seal with a different (non-BMW parts inventory) seal is a good idea. If you're going to go to all the trouble of drilling the hole put in a different seal.

NO way a seal should be shedding "little balls of material". I wonder if BMW got a bunch of bad clutch slave seals like they got a bunch of bad tire valves. If that is the case then replacing the just the seal or the whole slave assembly might result in putting back in another bad BMW seal. That thought might send me looking for an alternative seal source and rebuilding as long as all the other components check out okay.

I have rebuilt several clutch and brake master and slave cylinders off 1970s vehicles and have never see a seal failure such as Jeff describes. Corrosion of the cylinder, scoring of the seal, etc. I have seen. I have rebuilt several BMW airhead front brake master cylinders using the Original Seals when the seals looked good. These are bikes that have been sitting for decades without brake fluid change, in one case the brake caliber cylinders were seized, and the brake fluid was cloudy mud. Complete disassembly, clean up, and reassembly with the original seal in the master cylinder and it is working fine. If a 1970s BMW can do this, a 2000+ year model should have a clutch cylinder that never needs replacing. Yup, my bet is that some of the seals in K1200LT clutch slaves are of bad composition and that is why they are failing.

Like the tire valve situation, there is no telling how many defective clutch seals and clutch slaves with bad seals are distributed throughout the BMW parts system.

Crown wheel bearings, tire valves, clutch slave seals.... these aren't BMW engineering problems. These are quality contol issues with BMW contract suppliers. (I know that is true with the tire valve situation, I am betting it is the case with the crown wheel bearing, and now I strongly suspect it is the case with the clutch slave seal).

BTW, my 2000 KLT has over 50K miles, no final drive failure, no clutch failure, the tire valves were defective but I disovered before any failure thanks to some posting about the problem. Love the bike and plan on keeping it indefinitely.
 

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Discussion Starter #8
Dave,

That is a good point regarding the bearing. I wonder what the failure rate of the seals is vs. the bearing. I also wonder if the bearing fails because it becomes saturated with brake fluid. I'm assuming the brake fluid would be a poor lubricant compared to the grease originally in the bearing.

Kevin
 

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If the seal is made from rubber, I blame it on German technology (no offense to anyone). But it seems that who ever making the bmw parts has some defective rubber. This is why so many rear main seals are leaking, cheap rubber! The rear main seal on my LT solidified, which crumpled in place and created a leak. My LT only had 60k miles at the time with me using synthetic oil since 12k.
My $.02
 

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Hmmmm

jeffklt said:
BTDT I dismantled two and found that the seal sheds little balls of material as it deteriorates.
I believe that is a (the?) clue.

The deterioration you describe is indicative of contamination from petroleum products. The rubber used in the manufacture of brake parts is not compatible with oil. (And vice versa.) The material becomes "fatty" and abrades very easily leaving all sorts of little pieces behind.

Makes me wonder if the problem of failed cylinders actually starts because of a faulty transmission shaft seal. It takes very little oil to degrade a brake seal. Might also explain why some people have had a second failure in relatively short time.

Also sounds like another good reason to drill the transmission housing drain hole.
 

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hoodoodrum said:
If the slave cylinder fits neatly into the slave port, it appears there is no room for fluid to escape even if we drill an escape route. See my pics below
It is not tight, there is room for fluid to flow around the nose of the cylinder.
 

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My buddy Paul found the info and sent me the following:

I finally got around to the seal on your BMW. Actually I cleaned my shop yesterday and found your slave cyl . This a U cup, N14 x 20 x 4. Metric. Nothing rocket science about it. I am substituting polyurethane instead of the nitrile rubber they used. The polyurethane will last a lot longer but will have a skosh more friction. You wont’ notice the different at the low pressures and 20 mm bore in this rascal.

He is going to get me some pretty inexpensively so if anyone is interested in them I will post again when they are received.
 

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Sure.. if it works out, I'd try it.

To verify operation, it would be easy to fill with fluid and pump up the pressure..

Does anyone know how much pressure is required to operate the pressure plate?
 

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jeffklt said:
My buddy Paul found the info and sent me the following:

I finally got around to the seal on your BMW. Actually I cleaned my shop yesterday and found your slave cyl . This a U cup, N14 x 20 x 4. Metric. Nothing rocket science about it. I am substituting polyurethane instead of the nitrile rubber they used. The polyurethane will last a lot longer but will have a skosh more friction. You wont’ notice the different at the low pressures and 20 mm bore in this rascal.

He is going to get me some pretty inexpensively so if anyone is interested in them I will post again when they are received.
Has he checked the chemical compatability of the urethane with brake fluid? I just did a search and came up with this:
http://www.marcorubber.com/materialguide.htm

Note that urethane is NOT recommended for automotive or aircraft brake fluids.
 

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dshealey said:
Has he checked the chemical compatability of the urethane with brake fluid? I just did a search and came up with this:
http://www.marcorubber.com/materialguide.htm

Note that urethane is NOT recommended for automotive or aircraft brake fluids.
Thanks for the link David, what I find particularly interesting is that the current part is Nitrile which also is NOT recommended for automotive or aircraft brake fluids. I forwarded the link to Paul and await his comments.
 

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jeffklt said:
Thanks for the link David, what I find particularly interesting is that the current part is Nitrile which also is NOT recommended for automotive or aircraft brake fluids. I forwarded the link to Paul and await his comments.
My bet would be that the original seal is EPDM. (Ethylene Propylene)

Here is another chemical resistance source, with many mentions of brake fluid.
http://www.pspglobal.com/polymers-elastomers.html
 

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Discussion Starter #17
I did some reading on seals and the u-cup seals are made to seal using the hydraulic pressure to push the seal agains the cylinder wall. i.e. the seal is energized by the hydraulic pressure.

If the seal isn't energized then it can leak. Is it possible that the seals are slowly leaking when the clutch is at rest? There are also pre-energized seals with o-rings inserted in the groove. Would this potentially be better?

The EPDM rubber is the correct material. I will see if I can test the original seal in the slave cylinder but I'd be surprised if it wasn't EPDM.

Kevin
 
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