Gotta be an air bubble. Nothing else I can think of. Any ideas for how to get rid of it?

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Finn John -- Albany, Ore.
www.offbeatoregon.com
76 Merc 300D, sky-blue, *86k, for driving
84 Jaguar XJ6, black, 245k, for restoring someday
71 Merc 408, 2.3 petrol 4, Avon caravan conversion (UK), RHD