Thanks for the support, moral and otherwise.
The pump is new, not reman. I sourced it from my usual supplier - Rusty Cullens. He sells out of the Worldpac catalog, so it's the same stuff available via Fastlane. I forget whether the pump is Laso or Graf.
I too am beginning to wonder if the pump is defective. I'll check more closely when I pull it out again. Will examine for cracks, defects, and flatness of the mating surface.
I'm very much aligned with all the advice given here. The pump has dowel sleeves on the holes which allow it to hang in place before any bolts are inserted. Hence there is very little movement or disturbance to the o-ring. I'm tightening the bolts in steps, and not overtightening. (A torque wrench isn't accurate when using a swivel joint, plus one of the bolts on the m104 requires use of a standard allen key - no room for a socket, with or without swivel, FLX13, or whatever.)
On the plus side, I'm getting really good at this job. And I replaced the broken tensioner adjustment bolt thingie. So we're making progress on the little things, just not the big things.
'93 300E 2.8 91K (dead)
'87 300TD 132K (alive)
'98 E300 54K (gone)