Did you loosen the round knob (the primer pump handle) before you started pumping it? I'm not absolutely certain, but I think all the pumping will be in vain if you do not open it up (turn counterclockwise). If it started once and then failed to start again, it surely sounds like air in the lines. I've needed to prime mine numerous times when fooling with the fuel system and have always been able to pump it up, rather than cracking lines at injectors. Maybe I'm just lucky, but so far a little pumping (then retightening the pump handle) did the trick. Of course filling the large filter with fuel is highly recommended. Good luck.

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John Schroader
bio burnin' 83 300D, '83 300 SD, '79 240D
"I've never met a man who was good at making excuses who was good at anything else" Ben Franklin
"You cannot permanently help a man by doing for him what he could and should do for himself" Abraham Lincoln