Hi Duane (and Bill),
Yes, I am thinking it is the manual priming pump and not really the low pressure pump. When I saw the comment about Mac Boring recommending changing the low pressure on the Yanmar it caught my attention--thinking that it would cause fuel starvation or problems with the HP pump or leaking.
I had a low pressure pump failure (it is separate) on my Santorin's Perkins M50 and leaked through a weep hole designed to ensure that diesel fuel did not enter the engine's crank case.
Don't know if the integrated Yanmar low pressure ever fails but back in the day my Mercedes 220 D had a manual priming pump. One time the low pressure/feed pump failed but you could run the engine even at high rpm by hand pumping the priming pump. No way to test this but manning the priming pump might get you into port!
Shout out---Duane, Pat, and Bill K, et al--I've had really good luck with Yanmar and Onan parts up at Bay Shore Marine at Bert Jabins Yacht Yard. They are also good about shipping parts. They market a really nice impeller puller pliars that has worked very well on the Yanmar and Onan.