Why does clutch fluid turn black so soon. - Page 2 - Ducati.ms - The Ultimate Ducati Forum
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post #11 of 38 (permalink) Old Feb 8th, 2010, 2:13 pm
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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!!!
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post #12 of 38 (permalink) Old Sep 22nd, 2010, 6:32 pm
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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.

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post #13 of 38 (permalink) Old Sep 28th, 2010, 10:00 am
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None of my bikes with aftermarket slave cylinders have dirty fluid.
-M
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post #14 of 38 (permalink) Old Sep 28th, 2010, 10:26 am
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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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post #15 of 38 (permalink) Old Sep 28th, 2010, 6:16 pm Thread Starter
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None of my bikes with aftermarket slave cylinders have dirty fluid.
-M
Now that is interesting.
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post #16 of 38 (permalink) Old Jul 11th, 2016, 6:58 pm
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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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post #17 of 38 (permalink) Old Jul 20th, 2016, 3:05 pm
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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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post #18 of 38 (permalink) Old Jul 28th, 2016, 3:51 pm
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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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post #19 of 38 (permalink) Old Aug 8th, 2016, 9:30 pm
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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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post #20 of 38 (permalink) Old Mar 16th, 2017, 11:55 am
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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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