I had air in pipe before, it would come and go every now and then and did not appear to get in way with working engine. This day I was changing my filters and after the job, before changing fuel filter, it appeared that air had got into fuel line by larger amount. Okay suppose the o-ring on OUT pipe connection wasn't air-tight. After I bleed fuel line and went over all pipe connections, it hasn't shown any bubbles. Work better than ever now and I am glad to have went DIY-path on regular service. Cabin air filter could have been of lesser PITA though.What made you open up the fuel line in the first place? Doing that on any diesel is a sure recipe for a Big Job. Congrats on a quick resolution though!