Happened to me before when changing secondary filter.
While priming, did you see return flow?
Disconnect the cigar hose and make sure there’s something coming out.
You may have a big air bubble in the IP.

On that note… are you parked nose up?
For whatever reason, when I do any work to my IP, unless I’m parking nose down, I have a bear of a time starting again.

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1981 240D 4sp manual. Ivory White.