So I've got an odd clutch problem. The bite point basically moves around, typically in, up to a point. After the clutch is let back out, the engagement point will gradually migrate its way back to where it was originally, as evidenced by the free play in the lever. Another way of saying this: set free play to ~2mm, pull the clutch in a bit, let it out, free play is now 6mm. Leave it alone for ~10 seconds and the free play is now back where it was originally set.

I've put a master cylinder rebuild kit in it once already, and the problem seemed to improve, but it's now back to being as bad it was originally. I've got a new master cylinder coming. I don't see evidence that the bore in the old one is actually flawed somehow (and I don't see evidence of a burr or similar, and the seals on the piston don't appear knicked at all).

So until the master shows up, given the behavior and the results of changing out the piston once, what are bets that it's going to fix it, or do you think it's the slave? I've seen no signs of fluid loss.

Edited May 30, 20197 yr by heffergm

I had the same problem with a clutch master cylinder on a 2017 Evo.

The piston was sticking in the master cylinder bore, I had to remove the hose and use a screwdriver to push it out.

I put a new piston kit in which lasted about 9 months until the same thing happened again.

I suspect that the rubber piston seals are swelling up over time and causing the piston to stick, as there's no evidence of damage to the metal part of the piston or the master cylinder bore.

I’ve just replaced the master cylinder with a new one which is working fine.

Interesting... my new piston lasted a few weeks. It definitely does seem to be doing what you say though, sticking.

What brand of bike are we discussing?

'19 Sherco. It's an AJP master. I'm working through this with the dealer obviously to get parts, but narrowing things down might save some time.

15 hours ago, dabba said:

I suspect that the rubber piston seals are swelling up over time and causing the piston to stick, as there's no evidence of damage to the metal part of the piston or the master cylinder bore.

 

Possibly the wrong seal for the fluid?  The seals for DOT4 versus mineral oil are different.  If the wrong seal for the fluid is used, swelling can result.

Unlikely, unless both the factory and the replacement were the wrong piston kits.

Well, it's not the master. New one on, same behavior. Slave rebuild next...

On 5/30/2019 at 4:35 PM, heffergm said:

'19 Sherco.

Am assuming diaphragm clutch.  Gotta say, I'm not a fan.  In my experience, seemingly small dimensional changes cause large changes in operation.  I admired Sherco for sticking with what worked for so long.

 

Well, wasn't the slave either, or at least not the slave piston. No signs of scoring in the slave. Baffling. Unless it's possible to get a bad master cylinder, a bad rebuild, and another bad master. The likelihood seems low...

I'm assuming the diaphragm clutch Shercos use mineral oil in the hydraulic system, so this thought is predicated on that being the case.  If you're seeing a swelling of seals, this indicates a cross-contamination of fluids.  Historically, Sherco used DOT4 in the clutch hydraulics.  If, at some point in the bike's life, someone inadvertently put even a small about of DOT4 into a mineral oil system, I could see it causing symptoms like what you are experiencing.  If it were me, I'd do a complete flush of the hydraulic system if I found swollen seals. 

I've also attached a page from Sherco's 2019 press release.  Looks like they changed a lot since the prior model year.   Can you find a specification for the clutch pack thickness?  What does yours measure?

Edited June 8, 20197 yr by konrad

Unless it shipped with dot4 in it, which is unlikely, it's only ever had mineral oil in it.  

Clutch pack would not be relevant to the symptoms.

Edited June 8, 20197 yr by heffergm

A long shot, but you tried both ends, so now try what links them..... Switch out the clutch hose, as there could be some internal manufacturing flaw in the line.

I'll suggest one final thing.  I have a Clake OLC master cylinder.  The free-play adjustment on that is critical.   There's almost no range of adjustment between the clutch not slipping and it disengaging completely.   And it can exhibit similar lever effects to those you mentioned.

 

 

10 hours ago, nhuskys said:

A long shot, but you tried both ends, so now try what links them..... Switch out the clutch hose, as there could be some internal manufacturing flaw in the line.

You might be right . I had a problem on a gas gas years ago where it would get air in the clutch system overnight. Pump the lever and it would return to correct bite point. As the hydraulic line is covered in a protective hose I suspect it was weeping or damaged somewhere but you couldn't see it. Changed the hose and problem dissapeared. 

Other thing to check may be the clutch basket itself. The centre bolt on a gas gas can come loose and push against the slave, so the lever goes solid but the clutch doesn't disengage. Not sure how the basket is held on as Sherco design may be different but worth checking.