Marin postulated: You don't say if this particular Racor is only in the fuel polishing system or if it's also a (or the) filter for the engine itself. If it's in the fuel line to the engine and there are no bubbles in it when the engine is running, and you have not had any problem with air getting into the fuel feed to the engine (which you say you haven't), then I would suspect the bubbles are coming from some component of your polishing system. Scott adds: Marin, you are spot on! The filter is dedicated to the fuel polishing system and feeds the selected tank, so my risk of supplying air to the engine is zero. There is risk of loosing prime in the suction pump (I believe the pump is sucking fuel through the filter, not pushing it through. So, if enough air accumulated in the filter I guess it could cause cavitation in the pump. I'll pull the top soon and look. Thanks for all the hints and advice. Scott