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Clutch slave cylinder seal?

Discussion in 'Technical and Troubleshooting Torque' started by titus, Jul 14, 2014.

  1. About 9 months ago I replaced the original KTM clutch slave cylinder (a known troublemaker) with a dual seal Oberon replacement unit. It worked fine until the last month when I realised that I need to 'pump up' the pressure with the leaver each time I get on. It also goes a bit soft after 10min or so of cruising, until the lever is worked a couple of times.
    There is no sign of an external leak and when I cracked it open and bled it there were no air bubbles and it seemed to be full to the brim with fluid.
    I'm thinking the only thing left is fluid getting past the piston seal in the cylinder while at rest overnight, but the manufacturer (who is trying to be helpful) has never experienced that kind of failure even after selling thousands of similar units.
    Before I rip it out and ship it back to the UK for a replacement is there anything else I can try?

  2. If there was a leak your reservoir fluid level should be dropping. If this isn't happening I would suspect air trapped somewhere in the line.

    Is the line a single continuous hose or are there some joins along the way? If not continuous maybe you could try bleeding at the unions.
  3. Sometimes when the fluid breaks down, gas bubbles form in the system. Most of them self bleed through the master cylinder, but some can stick to the surfaces and be difficult to dislodge sometimes. One trick is to hold the lever in, so that there is pressure in the system, and then proceed to tap the hose & slave cylinder with a plastic screwdriver handle, attempting to dislodge the bubble. You may need to lean the bike over so that the bleed nipple is at the highest point, so the bubble will collect there.
    • Informative Informative x 1
  4. The fluid in this case is mineral oil, not traditional brake fluid so I would be a little surprised if broke down into, or absorbed, gases. However, not impossible so we'll give it a try.
    Seems to be getting worse.
  5. #5 коннор, Jul 15, 2014
    Last edited: Jul 15, 2014
    Did you bleed it the correct way (pushing fluid down from the top, adding more through the reservoir), or did you backbleed (pushing fluid up from the bottom using a large syringe of fluid)? While the former has advantages when it comes to getting crud out of the system, the latter can be more effective for getting rid of bubbles, so I would suggest taking that option if you didn't the first time and end up bleeding it again.

    In terms of alternative possibilities, 'bout all I can think of is something stretching and very slowly returning to shape. Seems a bit of a stretch, though, if you can excuse the pun.

    EDIT: Another trick is to grab a cable tie and leave the lever pulled in over night, then slowly release it. The idea being that the higher pressure breaks bubbles ups/compresses them, and the lower surface area of the smaller bubbles makes them less inclined to stick to things and will therefore cause them to slowly make their way up to the piston. Might be good to combine it with the tapping idea.
  6. The cable tie trick has certainly been known to work. I'd wrap the grip so the cable tie doesn't coy in.

    Any sign of fluid loss yet?
  7. Yes KTM recommend "mineral oil" in their brake & clutch. But from research it appears this "mineral oil" is nothing other than DOT4 brake fluid. I guess they just say that because they don't want you to use DOT5, which is silicone based, or DOT5.1 which is usually synthetic based. It does not mean you use mineral engine oil.

    The reason I suggested gas bubble formation, is that I have seen it a number of times on bikes that have been left sitting for a while, the brake lever becomes spongy, and a bubble is found in the calliper. Prior to that the brakes were fine, and with no leaks, I could only assume gas bubble formed from some chemical reaction in the fluid.

    Your situation is a little different though. Happening at a faster rate.

    If fluid is not leaking out, then perhaps air is leaking in somehow? I have seen air being sucked into a master cylinder past the piston cup seal when the lever is released suddenly. Perhaps the master cylinder seal is getting a bit tired.

    Also have you actually tried pulling the slave cylinder off & looked for a leak around the piston? If there is no leak, then while you have it off you could try pushing the piston all the way in. You might get lucky and flush a bubble out.
  8. Hmmm. I'm think you may have something in regard to the master. It's really quite unlikely to be the seal in the slave, all things considered. I'll have a look at both as you suggest on the weekend.
  9. @titus@titus I did some googling on your issue and came up with a lot of interesting reading!

    Firstly, brake fluid vs mineral oil. Read this, the member "evoluzione" was a rep for Evoluzione Cyclesports. 38580 Celine Cove Suite B Murrieta, California 92562 who made an after market replacement similar (maybe) to the Oberon unit you mention. Anyway, the point is, there is a lot of discussion about what is meant by mineral oil in this application, and "evoluzione" had this to say;
    yes, ktm and bmw use mineral oil that is not compatable with brake fluid. as to atf, yes it will work but it is a little "heavy" (~7.5 wt vs. 0-2.5 wt for mineral oil).

    However, Evoluzione Cyclesports appears to have now gone out of business.

    In other threads on ADVrider, others report using fork oil as a suitable replacement. Note the oil weight comparo in the quote above.

    This also gets a mention, http://www.penriteoil.com.au/products.php?id_categ=9&id_subcateg=120&id_products=159

    Secondly, in some of the threads there was mention of distortion and wear in a plastic spacer where the slave cylinder mounts to the case. Would this have any relevance to your case? http://www.advrider.com/forums/showthread.php?t=276463

    Hope this helps.

  10. Thanks for the info Gunissan.
    Last thing first: I replaced the original plastic slave cylinder mount with an Oberon alloy one at the same time I changed the cylinder so that possibility has been covered off.

    Evolutione products seem to be available again on some sites so perhaps they have been revived (?). FWIW there is also a third supplier (German, well respected) of upgrade slave cylinders called Sigutech.

    Re the fluid - yeah I could probably switch over to ATF although I don't know if that is the actual cause of my issue this time.
    After thinking about Tinkerer's comments, it's got some hallmarks of master cylinder seal failure to me. But we'll see.
  11. Well that was interesting. found a Youtube that suggested back-bleeding it through the master by pumping the shaft in the slave by hand. At first just a few tiny bubbles, but they didn't stop coming and as it went on (half an hour) they got bigger and just kept coming. And coming. And coming. Until finally it sorta petered out. Got no feeling in my left hand from all the squeezing :(. :sneaky:.
    I'll know whether it's done the trick in the morning.
  12. If you got rid of that much air I suspect here should be some improvement.
  13. Sounds like it was a candidate for reverse bleeding all along! For what it's worth now, using a syringe full of brake fluid and tubing connected to the bleed nipple on the slave cylinder and forcing fluid back towards the master is what this means. Supposed to be sometimes better than conventional master>slave bleeding, especially when there is an up and over loop in the fluid lines as on many dirt bikes. Said to be better at removing stubborn air bubbles trapped at the high point in the lines.

    Another variation on this bottom up method is by removing the master from the bars and raising it up as high as the lines allow, eliminating the loop in the lines to encourage the bubbles to float up towards the master, and then operating the master several times without opening the slave bleed nipple, the release strokes allowing the air bubbles to vent back into the master reservoir.

    Best of luck.

  14. It held pressure overnight and this morning so looks to be okay for now. I think I'll do a full flush and replacement with syringe sometime soon though. Going from the bottom to the top definitely worked better as per Tinkerer's idea.
    There's no up-and-over loop in this case and it's still a bit of mystery where the air came from (chemical breakdown maybe or the master cup seal, as suggested?)

    Was not fun stalling in traffic on the way home last night!
  15. Good to see you have had some success. Sometimes stuff just happens, with no plausible explanation.
  16. I'm betting your master cylinder needs a kit.
  17. How much will you win? I ordered the kit an hour ago :LOL:

    Yes, it's still losing a little bit of firmness at the lever. Better than before but still NQR.
  18. I'll settle for a kiss.