The o-ring is, and should be, the only seal needed. Check for a silicone goober you might have missed or perhaps the o-ring slipped out of its groove and got pinched during assembly.

Good luck,
Jim

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2005 C240 4matic wagon (daily driver)
87 190D - 225K (on loan)
85 190D - 312K (on loan)
2011 Subaru Legacy AWD (Wife's)