What an issue looks like:
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My rear cylinder at 55k miles. Bone stock. Being replaced under extended warranty this week.
My experience is atypical.
I'd guess 90% or more owners never have a problem, but that does not mean that problems can't happen. I'd say 90% of 4runner owners never post on this forum, either. No warning light yet. It may be clunking on left-hand turns, or I might be hallucinating.
It's just a leak, anything can leak. Only Toyota knows the actual vs predicted failure rates.
Have you ever had an issue with KDSS on your 4Runner if so equipped?