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About a month ago, I bought a used 2009 Stumpjumper FSR similar to this. It was not used heavily and remains in great condition. Recently however, after medium to long rides, a black residue is left on the damper body of the rear shock, (A Specialized AFR Brain Shock 7.25x1.75").

picture 1 picture 2

If I try to remove the residue with my finger, it smears slightly across the damper body but doesn't seem to stain my finger like oil would. It's also fairly difficult to fully clean off. According to the seller, the bike received a comprehensive tune-up about a year ago including a re-grease/oil of the fork and shock and hadn't been ridden after that.

What might this residue be? And could it just be that the shock requires some new grease on the inside of the air can, or would it be advised to do a more complete air can service and replace the O-rings? (if so, what rebuild kit would work for this shock?)

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    it's possible that one of the o-ring/wiper seals is degenerating, perhaps due to contact with the wrong oil or due to age. it fits what you're seeing. has the shock had a service in the last couple of years? these seals would be replaced at service.
    – Noise
    Commented Apr 17, 2022 at 19:48
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    note that ''lube and tune up'' is not a substitute for a strip-down service.
    – Noise
    Commented Apr 17, 2022 at 19:50
  • IMO, it's bog-standard rubber residue. Commented Apr 17, 2022 at 22:00
  • @JoeK as far as I (and seller) knows, the shock only received a re-oil/grease. I'm not sure if the shock has ever been professionally serviced. I think you're right about the deteriorating seals, as the residue definitely seems rubbery.
    – Jacob B
    Commented Apr 18, 2022 at 1:53

1 Answer 1

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Hard to tell, as suggested in the comments, I suspect is rubber from deteriorating O Rings. Whatever it is, the shock needs a service. I would not delay getting this done as if the shock is out of oil/grease irreparable damage to the aircan can happen. Presume its beyond a basic aircan service and book in a full service and repair. I would make sure I took it to a shop that has the ability to repair a Brain (more specialized skills than an Aircan service).

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    Thanks! I think you're right, deteriorating seals would fit the fact that it's been sitting for so long. I wasn't sure if this was something to worry about at first, but I'm looking now to get it in for a service as soon as I can.
    – Jacob B
    Commented Apr 18, 2022 at 1:59
  • I agree that it's probably smeared and deteriorated rubber. If not for needing to have the shock serviced promptly, do you think this is probably soluble in rubbing alcohol?
    – Weiwen Ng
    Commented Apr 18, 2022 at 13:38

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