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Retired Pipe Polisher C2H6O+
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Why? And what is that black crud that settles in the bottom of the clutch fluid reservoir? The brakes don't do it and you would think that would suffer the worst duty. ???
 

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The fluid is in a closed loop. It should not produce any crud. Maybe try completely flushing out the line with new fluid and wiping out the reservoir. Also, check to make sure your slave cylinder does not have any leaks or bad seals which is pretty common. Good luck!
 

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I believe it's a reaction to moisture. Hydraulic fluid absorbs moisture. It may be from condensation inside the system, or moist air seeping in from outside the system. It happens quite a lot up here in Canada with our big temperature fluctuations. Just flush your system once a year and it will be fine.
 

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Retired Pipe Polisher C2H6O+
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Discussion Starter #5
But why only the clutch and not the brake. All my bikes do it and I've noticed it on other's bikes also.
 

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It's caused by aluminium particles dissolving into the clutch fluid, probably from the unfinished aluminium bore of the crap standard slave cylinder. Most aftermarket slave cylinders have anodised bores, or are made from a better grade of aluminium, so you'll notice that bikes with aftermarket slaves don't suffer from the problem.

BTW, it doesn't adversely affect the operation of the clutch, just looks bad.
 

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Retired Pipe Polisher C2H6O+
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Discussion Starter #7
It's caused by aluminium particles dissolving into the clutch fluid, probably from the unfinished aluminium bore of the crap standard slave cylinder. Most aftermarket slave cylinders have anodised bores, or are made from a better grade of aluminium, so you'll notice that bikes with aftermarket slaves don't suffer from the problem.

BTW, it doesn't adversely affect the operation of the clutch, just looks bad.
Thanks. Sounds logical. Your right, it's never been a problem, just looks nasty.
 

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Here's why. None of your reasonings make sense because as Duckman stated, why the clutch and not the brake?? Same components, same materials, same design. Except...

Unlike the brake system, as the disc in the clutch wear, (and along with the natural flow of fluid in the system) fluid is displaced into the reservoir. In a brake system, as brake pads wear and the piston is displaced further into the caliper, the fluid in the reservior decreases. With the clutch, the fluid in the reservior increases, bringing along with it, any contaminants, crud, or dust that happens to get into the system. If you'll notice, this is why you have a 'min' and a 'max' line on the clutch reservoir, but only a 'min' line on the brake.

We could argue how that gets into the system yes, but this is the reason it gets into the reservoir. If look closely however (at least best I have noticed on all three of my Ducs) the crud ends up in the reservoir stuck to the sides or resting in the bottom and not suspended in the fluid. I have never seen any detrimental effects from this, but I personally have never owned a Ducati that didn't do it.
 

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Actually the black stuff is the sealing ring in the slave that deteriorates and gets into the fluid. I took apart a slave and found the black ring to leave black crud on my fingers.
 

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The crud is as previously stated aluminum and rubber. The fluid darkens as it ages because it is hydroscopic and the water it picks up contributes to the corrosion of the cyl. bore.The reason it turns color so quickly is the relative distance the seals travel is about 4 times greater than in a brake cylinder.And the loading on the seal increases as it slides down the bore unlike the brake which stops moving pretty much at peak pressure.Just change the fluid once or twice a year if it gets too dirty.
 

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When I got my ST2 all the res's were discoloured, brake and clutch! So I ended up changing the fluid in all the systems. I think its down to the seals and also the rubber diaphram that sits in the oil all the time, I bet if they were a different colour then crud in the base of the res will also be that colour.

I did notice that my clutch one was worse, but I guess that is down to the volume of the system.

John
 

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Mr Leakered
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The crud is as previously stated aluminum and rubber. The fluid darkens as it ages because it is hydroscopic and the water it picks up contributes to the corrosion of the cyl. bore.The reason it turns color so quickly is the relative distance the seals travel is about 4 times greater than in a brake cylinder.And the loading on the seal increases as it slides down the bore unlike the brake which stops moving pretty much at peak pressure.Just change the fluid once or twice a year if it gets too dirty.
+1 The clutch travels the full range very often. The brake masters and calipers displace very little. So, there is very little fluid movement in the brake systems.

Have a good one.
 

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Tricklidz had the right answer... Fluid going black is the fried slave cylinder seal, other symptom is that your clutch bite point moves closer to the bar as you ride... After about 10 miles of town riding (I.e lots of clutch usage) it's virtually at the bar, yet if you leave the bike overnight, the biting point returns to normal when your first start the bike... The problem is the bearing on the clutch pressure plate. Over time the dry clutch wears and dust is created which gets into the bearing causing it to seize... Once that happens the the push rod spins with the clutch, this is turns spins the slave cylinder... Which gives rise to the two effect described earlier; namely black residue in the fluid (from the fried seal) and loss of clutch power when the spinning slave cylinder causes the fluid to boil. When it cools and condenses, clutch power is restored. Solution is to replace the bearing (a simple fix and costs about £10)
 

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I could also be that the fliuid is being heated because the throw bearing is worn or defective and the rod is spinning the piston in the slave!!!
My money is with tricklidz, that is why Ducati has gone with the pined push rod on all the latest slaves. It stops the rod spinning the piston in its bore.
Have you ever noticed when you sand aluminium with wet and dry paper how the water turns black? This is what's happening inside your slave.
 

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My money is with tricklidz, that is why Ducati has gone with the pined push rod on all the latest slaves. It stops the rod spinning the piston in its bore.
Have you ever noticed when you sand aluminium with wet and dry paper how the water turns black? This is what's happening inside your slave.
+1 I concur
 

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It's not just endemic to Ducatis guys.

Hondas, Suzukis and Kawasakis do it too.

I asked why back in the 90's at the big bike dealership.

We surmised that since the clutch master cylinder displaces so much more fluid, the dirty fluid was getting dragged back up into the master cylinder reservoir. Just from the rubber seal rubbing against the bore, the fluid turns black from aluminum residue.
.
 

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Whatever the case, it seems to be true across not just different bikes but different manufacturers. My Moto Guzzi and Duc have had the same problem, as did my old Suzuki. Sounds like a case is being made for the aftermarket slave. Having never had one I couldn't say for sure.
 
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