The delivery valve holder is off an '83 300D which I believe is the MW pump. The part I ordered from NAPA was a #6 O Ring Boss with a 1/4" NPT male end.

The delivery valve holder threaded into the ORB with teflon tape, but it did not tighten down hard, like the NPT fittings which I believe have a taper. I had asked the guy at NAPA when I picked up the ORB if I needed an O ring and he said no. Guess he was wrong.

The fitting leaked the first time I tried the tester, so I went to the hardware store and eyeballed a couple of O rings and one of them fit. Inserted the O ring in the boss, tightened the fitting and it is now leak free.

Now if I could just solve the slight seep that I have where the big center nut meets the jack body, the tester would be all buttoned up. I wonder if that metal on metal fit is meant to hold fluids (like diesel), especially when the reservoir is higher than the joint.

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I built my pop tester using a port-a-power hydraulic pump. They can be bought separately and fittings easily connected via NPT thread. All said and done, I maybe spent about $130 in parts. I have a port-a-power but I'm not about to fill it with Diesel after what I paid for it, so I got a separate pump for something like $80 without any of the accessories.