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Discussion Starter #1
Ok ... I am starting to get a little frustrated. I always have to bleed my clutch after about 200-300 miles of street riding, or in the middle of doing a trackday when I am doing a lot of continuous up/down shifting.
I don't get any air from the bleeder screw at the slave cylinder when bleeding, however, when I bleed the mastercylinder I get just a few bubbles (One or two cycles on open/closing of the bleeding port inside the resevoir) and then it is good as new.

I know for a fact that I do not have any leaks from anywhere on my hydraulic system for the clutch. I have replaced the mastercylinder with a new OEM part thinking that it was allowing air to get past the seals somehow, but the issue still persists. Logic tells me that in order for air to get into a sealed hydraulic system, then fluid would have an avenue to leak out while under pressure. I have repeatedly and thoroughly inspected my mastercylinder, line, and slave for any signs of leakage and all appear to be in proper operating condition with no leaks of any kind. I am pretty technically inclined, but this has me stumped. It really does not make any sense.

Can someone shed some factual knowledge as to why I keep getting air in my system? Having to constantly bleed my clutch hydraulics is getting old, and I would like to find out how to alleviate this once and for all.

Thanks in advance for your replies.
 

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Have you tried injecting brake fluid from the slave cylinder back up the clutch line? Sometimes I have had troubles getting all the air out of the system. I use a syringe and inject the brake fluid from the slave as air always tries to rise to the top.

Mike
 

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Mayor of Simpleton
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Sounds like the master cylinder, but you say it's been replaced. Did you use new crush washers all around? High quality DOT5 brake fluid?
 

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Mayor of Simpleton
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Actually, if all the other stuff is good, I bet your clutch line has failed right up at the top banjo where the fitting is crimped to the tubing.
 

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One thing to think about is that the piston seals in the master and slave are designed to resist the outward pressure of the fluid, but when the pressure is released, and for example, the slave piston is drawn back to it's rest position, the seal is not being pressed against the bore by the fluid pressure. Any out of roundness in the bore would allow air to be drawn in. The slave bore in my bike had a very obvious spot where the seal was not making good contact with the bore. A few turns with a cylinder hone might fix it.
 

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Old Wizard
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Flush

Your problem is boiling hydraulic fluid, or more correctly, absorbed water in the hydraulic fluid is boiling

The engine temperature at the slave cylinder location can easily reach 212 degrees F (the boiling point of water), and nearby small pockets of water, as they boil, introduce water vapor in the line that causes behavior just like air in the line does.

The amount of water that hydraulic fluid can hold in solution is dependant on the temperature of the fluid. So, The Ducati clutch hydraulic circuit is a little water generator. When the fluid heats-up from engine heat, it can hold more than five times more water than it could hold at room temperature. Moisture in the air usually enters through leaks in the reservoir cover and gets absorbed into the fluid.

Then the fluid cools down when you shutdown so the additional fluid that could be held at the higher temperature, condenses out into water droplets that sink to the low point of the system - to the slave cylinder. This happens over-and-over until enough water has accumulated at the hottest point in the system. At 212 degrees F it boils and your clutch actuation goes south.

So flush your system completely to get rid of all of the water and fluid. Then refill from a new sealed container and check to make sure your reservoir cover seals properly.
 

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Discussion Starter #7
Thanks for the replies.

The fluid in the resevoir is new, and it has been flushed several times to make sure that I had absolutely no air in the system.

I have a hard time buying the idea that I could draw air into the system (low pressure) through banjo bolt fittings or a poorly sealing crimp fitting, and yet not have a leak (high pressure) in the same area. That just is not logical to me.

As far as the moisture boiling in the fluid, I can totally see that happening if the fluid were old and not maintained properly. However, as stated above, the system has been flushed several times with new fluid and the symptom remains. Under most mastercylinder failures that I have encountered, the pressure bleed-off through the internal seals usually causes a "spongy" feel at the lever, but I have never seen air bubbles in those mastercylinders when I have bled them as part of a diagnosis.

So ... I guess my question appears to remain unanswered at this point.
 

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Mayor of Simpleton
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Or, if the master cylinder and fluid are new, then it's still a very small leak in the line.

or elves.
 

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Old Wizard
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If you have water in the system it will pool at the lowest point in the slave cylinder and will be very, very difficult to remove using usual bleeding techniques. I suggest removing the slave, remove the water and refill. Or not.
 

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Chilehead
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A) Which bike?

B) If it's a xx9, it's a known problem, and is NOT water in the system nor fluid boiling. Don't know where it sucks it in from but it does. I do a full bleed once a year, using different colored fluids, and inverting the slave to make sure it is fully drained. I have three Ducatis and two Guzzis with hydraulic clutches (the Guzzis are obviously not OEM), only the 999R has this problem.

C) It is not correct to think that just because you can suck air in under negative pressure, fluid will leak out under positive pressure. Air and fluid do not have the same viscosity, and seals do not have the same sealing properties in both situations. That said, if you were to drain the system and pressurize it with only air and find your leak, but then again not as the seals may not seal in the absence of fluid.

Tom
 

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Discussion Starter #11
So ... I was told by a reputable shop here that the ONLY way to alleviate this issue is to replace the OEM clutch mastercylinder with an aftermarket Brembo unit.
Apparently it is a well known issue with the 749/999 series bikes, and there is really no explanation as to why the clutch mastercylinders are causing this issue. From their experience, the only way they have been able to resolve the issue is to replace the mastercylinder with an upgraded Brembo unit.

WTF?
How is it that Ducati can have a common issue like this with only the clutch side of the bike and not the brake side? The mastercylinders are basically the same design strategy ...? It is really frustrating that someone should be forced to "upgrade" a component just so that a system will perform as it should on the bike.

Anyway ... has anyone had the "Constant-air-in-the-clutch" syndrome been able to completely alleviate it by replacing the mastercylinder with an aftermarket Brembo unit? I don't mind upgrades, but I can't justify spending the $350.00+ just so I can have aftermarket "bling" that has no functional performance gain.
 

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Humor me and flush the system thoroughly.
+1

I went through this on an MV Agusta. It would always feel mushy after it was ridden.

When we swapped the calipers out, we found water in the stock calipers. We had flushed two bottles through it in the last month. We thought we would have got it all out during that time. We were wrong.

-M
 

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Discussion Starter #14
Humor me and flush the system thoroughly.
As stated previously, I have flush the system several times.
I even did it the day before my last track day, and I still end-up with the same results. So, you should be laughing your ass off right about now. :D

Believe me when I say "I know what I am doing" when it comes to flushing fluids (I am a BMW tech by trade, and very mechanically capable), but I am frustrated. Especially when a reputable shop like BMS tells me the only way to alleviate a common issue with a specific model line-up is to "upgrade" a component. :think:
 

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Old Wizard
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The problem is not with your experience or technique, it's the configuration of the system. Water is heavier than hydraulic fluid and sinks to the bottom of the system, the slave unit. The bleed valve is mounted above the slave cylinder bore so no amount of flushing will remove the water below. You have to remove the slave and flush it, reattach the line and refill.
 

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Discussion Starter #16
The problem is not with your experience or technique, it's the configuration of the system. Water is heavier than hydraulic fluid and sinks to the bottom of the system, the slave unit. The bleed valve is mounted above the slave cylinder bore so no amount of flushing will remove the water below. You have to remove the slave and flush it, reattach the line and refill.

Ok ... I grasp that concept. I will give that a try this weekend and see what happens.
So, I should be able to remove the slave, turn it upside down, and perform a flush using a vacuum flushing device (a Vacula) and acheive the same effect as what you are describing ... correct?
 

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Ok ... I grasp that concept. I will give that a try this weekend and see what happens.
So, I should be able to remove the slave, turn it upside down, and perform a flush using a vacuum flushing device (a Vacula) and acheive the same effect as what you are describing ... correct?
Durbahn in german make a cable clutch conversion for a number of Ducati superbike models including the 749/999 series bikes. (They have an english online shop)



It's not cheap by any stretch of the imagination, but you can aways sell your stock parts to cover some of the cost.
 

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My 10p's (20c) worth - 3 aftermarket slaves/aftermarket reservoir/3 gazillion bleeds later - still getting air in the system within 200 miles.

I like Shazam's explanation (the slave does get REALLY hot). However, I have tried the "back bleed" procedure several times - eg: remove the slave, use g-clamp or similar to push the piston all the way back in the bore, refill from the master. No bleedin' good (sorry).

At the track I tie the clutch lever back to the bar between sessions - seems to help, or maybe it's just sykolojikal.

Now, where did I put Durbhan's number......

BTW - 998, and the aftermarket slaves are all Evoluzione's.

Spin
 

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Chilehead
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Having water in the fluid will only explain air in the system while it is hot (i.e. past the boiling point), not why it is there once the system cools. The water vapor returns back to liquid water when it cools, and thus CANNOT be the air in the system when cold!

Tom
 
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