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Premium Member
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Discussion Starter · #1 ·
A few weeks ago the clutch lever "collapsed" when I pulled it in to downshift. I was in a quick stop situation on a narrow bridge with no run-out and cars stopped in front of me and nothing happened when I pulled the lever...so I braked hard and juddered to an ugly ugly stop. When I pulled the lever again everything was normal. I rode back home without incident. Since then, here's what I've done:
Pulled the plates, cleaned, scuffed and rotated. Installed a new pressure plate, new stainless springs and retainers (speedymoto). Then, assuming the issue might be a weakened line or a faulty slave cylinder I installed a new slave & a new stainless line. Topped it off with fresh DOT 4 fluid and, or course, bled the line and adjusted the actuator.

I've ridden about 500 miles without incident and then a couple of days ago the same thing happened again...no resistance at all when pulling the lever leaving the clutch fully engaged and then a couple of pulls later and it behaved normally and disengaged. The lack of pressure, intermittent failure and recovery, and process of elimination leads me to suspect the master cylinder as the culprit but I thought I'd check with the ST~meisters on the forum to see if there is something I missed.

Another interesting thing, since I replaced the line I had to drain and add new fluid so while I was at it I bled and replaced the fluid in the brake reservoir/lines at the same time. The fluid in the clutch reservoir has already started turning dark. It's still clear but turning brown and noticeably darker than the fluid in the front or rear brake reservoir. What's that all about??


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Premium Member
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923 Posts
Possibly a Master Cylinder issue .......

Blocked compensator port ?
Perished seals ?
Blocked air bleed in reservoir cap ?


I'll bet that these intermittent clutch malfunctions occur ONLY when the engine is hot.
I don't mean warmed up in the driveway hot,,,, or even 10 minutes across town.

I mean 30 minutes or more of riding,,,, probably with not much airflow (city traffic type riding).
Time enough for engine heat to radiate through out all of the engine components ,,,, engine side cases, transmission, clutch etc, etc, etc ,,,, and then to the clutch slave cylinder,,,, then into the fluid contained within. Convection currents through the fluid of hot rising, cool falling, allows all of the clutch fluid to become heated. As soon as the first molecules of moisture become super heated to steam, the heat RAPIDLY disperses thoughout ALL of the fluid, heating and converting even more water molecules into steam. The point of change of state for the water molecules is almost instantaneous throughout the contained fluid.

Hence one minute you've got a clutch, the next you haven't ,,, then almost as quickly,,, it's almost back to normal again.

Brake fluid is hydroscopic ( attracts and holds moisture in suspension.)
If that moisture gets hot enough, the moisture boils, producing a gas (steam).
You can't compress a liquid (Pascals Theory), but you can compress a gas.
If there is moisture in your fluid, and there is sufficient engine heat transfer to your slave cylinder, that moisture boils off to become a gas.
You pull the clutch lever, pressure builds up proportionatly through out the fluid, but fluid transfer from the Master Cylinder to actuate the Slave Cylinder ceases at the point where the gas starts to compress.

That first pull on the lever gives you limited clutch travel,,,,, and the problem that you've experienced.

That first pull also pressurises the gas, which causes it to condense back to a liquid (just like a sealed food pressure cooker and a radiator cap raise the boiling point of the liquids within by increasing the pressure upon that liquid.

Subsequent applications of the clutch soon afterwards provides return of lever pressure and clutch operation back to almost normal because the gas has condensed back to an uncompressable liquid.


As to the darkening fluid ,,,, CONTAMINATION.
Somehow, somewhere, your fluid is being contaminated.
Which is further indicator to the condition I've described above.

Find the source/cause of the contamination, and you'll rectify the clutch malfunction.


Cheers
Trevor
 

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Discussion Starter · #4 ·
^^ both good responses, thank you...Brad, the pressure plate is new and I replaced the bearing at the same time. That doesn't mean it couldn't be defective but it does mean because it was new I did fail to troubleshoot that as a possible cause.
Latte...you're partially right, it has happened generally after 30 minutes or more of riding but in both instances I was traveling briskly on the open road not in traffic or in town driving. The first time I had just passed a string of cars (mostly legally) at speed and was downshifting coming into congestion. The second time I was also downshifting, engine braking, in the twisties. So temp was up, but not excessive and there was plenty of airflow. As to contamination, yes I agree the color of the fluid is a sure sign but damned if I can guess the cause. The entire system was drained when I replaced the line and slave. I haven't lost any fluid, the reservoir is only exposed to sunlight when I'm riding, but so are the brake reservoirs, and I filled all 3 reservoirs from the same container of fluid. Curiouser and curiouser. Thanks for the input guys, much appreciated.


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Super Moderator
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5,568 Posts
I can't say too much regarding the sudden loss of pressure...other then to agree with what Trevor has posted. The clutch fluid on my ST3 always gets dark pretty quickly. Everything works fine, just gets dark. I end up flushing the clutch fluid twice a season or about every 3,000 miles.

Good luck!
 

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A 748 Fanatic
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3,155 Posts
Fluid

The clutch fluid on my 748's do the same thing.

They get dark quicker than the brake fluids. :)

When I had this same thing happen on my 1998 748s, draining and replacing the fluid solved the situation and it has not returned to date. I figured it was moisture in liguid that turned to gas while hot as described here.

:)
 

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Premium Member
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Discussion Starter · #7 ·
^ thanks George. Last time I changed the fluid it was nast. I've had espresso that was lighter. I didn't have any problems after the last bleed/change so that makes sense. I have less than 2,000 miles since the change but I'm going to flush it and add fresh.
 

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My problems started after I had my clutch and brake fluids changed by the dealer. Within 100 miles my clutch master cylinder started failing. The failure was sparodic - and when it failed I had to keep pumping the handle to regain pressure. I took it back and had it checked for air in the lines before I replaced the master cylinder.

Now it has been 500 miles since the brake fluid change and my brake master cylinder seems to be failing. I will squeeze the handle about an inch before it engages but then can pump it back up to full pressure. It just seems a little unusual that both master cylinders would fail so soon after a fluid change. Maybe I just had my bike sitting around too long!
 

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Mr Leakered
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More likely that the lines were bled at the slave/caliper and not at (or completely) at the masters. Super easy to do. Either snag some banjo bleeders or wrap thoroughly with towels and crack them with the levers pulled in.

Have a good one.
 

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A 748 Fanatic
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My problems started after I had my clutch and brake fluids changed by the dealer. Within 100 miles my clutch master cylinder started failing. The failure was sparodic - and when it failed I had to keep pumping the handle to regain pressure. I took it back and had it checked for air in the lines before I replaced the master cylinder.

Now it has been 500 miles since the brake fluid change and my brake master cylinder seems to be failing. I will squeeze the handle about an inch before it engages but then can pump it back up to full pressure. It just seems a little unusual that both master cylinders would fail so soon after a fluid change. Maybe I just had my bike sitting around too long!
Maybe, just maybe the dealer has a large container that is often left open or or near a heat source during the day and cold at night and therefore condensing moisture inside and now you have his high prices new contaminated brake fluid in your system. just sayin, just maybe.

Use brake fluid new from the container and discard unused portion. By the time you need it again it will be no good if open and closed with air inside replacing the original brake fluid.
 

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Old Wizard
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3,007 Posts
is the bearing in the centre of the pressure plate good? pushrod spinning freely inside input shaft?
To expand on Brad's solution:

There's a common problem that mimics having air in the clutch hydraulic system. The clutch pushrod is spinning because the bearing in the pressure plate is not moving freely. This causes the pushrod to move the clutch slave piston back into its bore slightly.

So, when you ride for a while without using the clutch, when you try to use it, you have to pump the lever a little to first move the cylinder back into position, after which it works normally. Low-pull aftermarket slave units accentuate this problem because a full lever stroke moves the slave cylinder piston a shorter distance than it does the stock unit.

You usually just need to lubricate or replace the pushrod bearing on the pressure plate to cure the problem.
 

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Chilehead
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6,986 Posts
I had this problem with my ST2, it was the pump.

I bought a new one, plus rebuilt the original as a spare.

Tom
 

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The days are getting longer!
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I would also look at the master cylinder, its the only thing left. ;)
 

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My ST2 use to do this during "spirited" riding. (wait, ain't that ALWAYS?).
Once very occasionally, the lever would just go to the handlebar, without releasing the clutch. The only thing I touched, that made the problem disappear, was cleaning up the push rod, where it goes into the "derby hat"-- the clutch dust made it part of the rod. I also cleaned up the throw-out bearing. After doing this, it never re-surfaced.

My theory regarding the darkening clutch fluid is the rubber line. I went to a SS braided type, and installed a quad-ring in the slave. The other thing is, it's VERY dirty, mucky, and oily, near the front sprocket, and I believe some of that crud migrates into the slave.
 
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