Just leave it. The fluid tends to heat up and the black stuff is just tiny parts of the rubber coming into the fluid. It's normal with these clutches. Heck mine got pitch black dark after a small ride when I finished replacing the fluid.

As long as you're not noticing anything weird with the clutch, just let it be

 

The clutch fluid turns dark quickly because of the heat from the engine. The clutch slave cylinder and clutch push rod act like a heat sink collecting heat from the engine heating the fluid. A slightly larger reservoir probably won't help. The fluid in the brake reservoirs will not turn dark as fast because with a typical rider you don't use the brakes enough to heat the fluid. If you're an aggressive track rider the outcome might be a little different with the brake reservoirs. Just keep the clutch fluid changed with regular clutch bleeding.


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The clutch fluid turns dark quickly because of the heat from the engine. The clutch slave cylinder and clutch push rod act like a heat sink collecting heat from the engine heating the fluid. A slightly larger reservoir probably won't help. The fluid in the brake reservoirs will not turn dark as fast because with a typical rider you don't use the brakes enough to heat the fluid. If you're an aggressive track rider the outcome might be a little different with the brake reservoirs. Just keep the clutch fluid changed with regular clutch bleeding.

since I installed a quad ring in the (original) slave cylinder on the Monster, the fluid hasn't changed in color or opacity in the least. thousands of miles on it now. with the original seal, it would change within a few hundred miles. it's not the heat.

 

Did your clutch have asparagus for dinner?

 

old slave seal was leaking just enough to tick me off and require regular bleeding. finally had enough, took it apart, measured the piston ring groove and cylinder ID. then, I searched for the correct/closest quad ('X') ring and gave it a try. I haven't had to top up or bleed since (couple thousand miles and nearly a year) and no change in color or opacity.

I am pretty sure it was a -211 size (13/16" nominal OD) but, I haven't figured out the material. It is either Buna N or Viton. I'll try and nail it down tomorrow.

Side note: slave bore needed a touch of loving with a Scotchbrite flapper and still isn't perfect. But, the results belie the visual condition of the bore. Since I've never had an issue with Popeye arm syndrome, I've saved a few bills staying stock (and not leaking ).

 

That's some solid information SP3. Much appreciated.

 

I remembered to keep the original seal and it measures:

ID = 19.5 mm actual
cross section = 3.7 mm actual

-211 quad ring:

ID = 20.2 mm actual (13/16" nominal)
cross section = 3.53 mm actual

material hopefully tomorrow

 

confirmed the size and material: -211 in 70 durometer Buna-N (nitrile).

 

211 QUAD RING (X-RING) NITRILE 70 DUROMETER CS: .139" (3.53MM) ID: .796" (20.22MM)

 

that's it. it'll be interesting to see how long it lasts since it isn't a good choice for brake fluid. still, for 15 cents, I was happy to give it a go (in this application). so far, so good.

http://www.mcmaster.com/#o-rings/=rv181y

 

reviving this thread because I was rebuilding a slave cylinder for a friend today today and I remembered posting something about the quad rings. here we are 2.5 years closer to our demises and I've still not touched the clutch (hydraulics) and the fluid still the same color/clarity. just thought I'd give an update.

 

FWIW. The color comes from aluminum molecules being coaxed off the cylinders. Aluminum is a filthy material as anyone that welds it can attest. The reason why the clutch is dark and not the brake is the brake caliper piston makes contact in a short distance and prevents the master piston for moving much whereas the clutch pretty much strokes the whole bore on both ends with every use. The reason that the nitrile doesn't get dirty is because it doesn't seal as well and there is a lot less friction. It's always leaking a little although you won't notice it with most masters because of the secondary seal and/or the dust cap.

The correct seal to look for BTW, that is common, is made of butyl. Unfortunately, without understanding a lot about seal design and clearances, you shouldn't trial and error on the brake side but the clutch... eh. I made it work but it took some machining. Complete butyl O-ring sets are cheap. Trying to find a kit from Brembo would be the first step. There is a 50;50 chance that one that says it is right for your bike will fit, in my experience.

 

the stock seal design doesn't do a good job of keeping crap (chain lube, road grit/grime) out. that in turn accelerates the wear of the bare (un-anodized) bore. those molecules then oxidize and, along with the said 'crap', turns the fluid color. there may be some seal material worn off as well from the crap adding to the color.

the quad ('X') ring seems to keep the crap out, the bore is left alone, and the fluid is unchanged.

like I said, 2 and a half years without fluid level change or color change. in that time, I would have gone through a couple pints of fluid with the original seal design.

for the clutch slave, I won't be going back to the OE cup seal. the quad ring has already made it longer in use than the original did.

 

I searched for a EPDM Quad ring, before finding this thread. what a waste of time...
So, if NBR lasts that long, it needs a solution for the cylinder.
I think about to shorten the piston, press a POM insert inside, instead of the rod seal. that would give space for a 2mm O-ring to keep the rust out and silicone grease in.

 

The same seal, same cylinder still in place. I might have flushed/bled it once in all these years and that only 'just because'. NBR quad ring in a slave cylinder in decent condition and nothing more will be required.

 

ok, but my inner monk decided to do this while waiting for the rings

grinded on a lathe 1mm deep and parkerized. 1.5mm oring will fit perfect hopefully.