I have. found. the. problem.
Today I decided to take apart the HPFP to have another look when, you know, I
wasn't drinking.
I noticed that I hadn't taken off the piston head. So I proceeded to work on the four bolts, but the first one was uh, LOOSE.
WHA?! Loose as in a tiny gap under the bolt-head.
I took off the head and this is what I found: (click to expand)
The O-ring for the high pressure head had been carelessly installed, with no oil, and so the O-ring had been smushed.
Whoever it was just tightened it up and it lasted a couple years. When I went down the ledge with a -bump- it was enough for the final loosening of the bolt, the head leaked, and the engine quit in a classic mechanical failure syndrome.
Then I opened up the cover for the gear pump and that O-ring had also been smushed.
I'll bet I'm not the only one this has happened to.
DOH'Reilly Auto Parts only had fat O-rings and NAPTHA was closed, so it'll have to be Tuesday and a hydraulic hose shop to find decent O-rings.