Canon 100-400L IS Problem, black bits coming out of the sliding ring

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Update on my moans about this. I decided to 'play around' with the lens. Essentially I loosened the ring to its loosest point and then rotated the lens (focus) in the opposite direction multiple times. And then tried to tighten the lens-lock ring. After several goes I found a sweet spot which locks it absolutely tight (so tight I can't pull the lens out at all without loosening the ring). The sweet spot does work free in use over time, but now I know how to get it back it's usually just a quick fiddle to find it again. Not ideal, but nowhere near the problem I thought it was.