I've never rebuilt a 750 but I remember reading here that some folks even removed the pump to prime it directly,then re-installed it.
I have, twice within the past 2 weeks. I can't keep affording to buy 2 new gaskets every time I disassemble it. It's almost $55 for the 2 gaskets each time.
Don’t give up. Review the picture of the oil pump and oil flow. Make sure you understand where the two sections intake oil and where they discharge it. Build you a quick dog drive. Similar to a drill operated paint stirrer. Only the bolts you use for the pins in your dog drive will have to line up with two of the holes in your oil pump gear. Picture an all thread with a flat strap double nutted to one end. In that flat strap drill two holes equal distance from the center all thread hole. These holes will have to line up with two holes 180* across from each other on the oil pump gear. Put a long enough bolt in each hole that the ends of the bolt’s threads will engage your oil pump gear. You could put rubber tubes over the threads so you won’t mark up your gear. It doesn’t have to be perfect, just something the drill can spin the pump submerged in a pan of oil. It will prime quickly not having to push all the trapped air through the stopper valve then through the tight engine clearances. When you see two geysers shooting up from the surface of the oil in the pan, then you’re ready for a new gasket not before.
Before removing it I would still try cranking with out the gauge installed and see if oil shoots out. You could do the same with your oil filter off. Sometimes this may act like a manual grease gun you just changed grease tubes in. It’ll prime to the atmosphere but not hooked to the grease zerk with any back pressure on it. But we don’t know whether the stopper valve is used in “Sumpthing”?
You won’t be able to prime it through the oil gauge hole by squirting oil in it. This is the discharge side of the pressure section of the pump.
With a pressure bleeder maybe but I’ve never tried it and don’t know if it would damage the stopper valve. if yours still has one. I don’t know if you were required to modify that for your Sumpthing. I don’t see where the stopper would have a function because there is no tank on yours to worry about draining oil back into the dry sump. Because it’s a wet sump now..🤔
Is your oil gauge old school mechanical ? or part of the m unit.?
Below is a picture of the intake and the discharge ports for the pressure section. (Oil pressure) Observe the arrows.
Under the screen is the intake port for the scavenge section of the oil pump. This Scavenge section of the pump was used to return all pressurized oil drained back from the engine bearings, camshaft, sling oil, ect back to the oil tank. The scavenge section is always trying to keep the sump dry..hence dry sump. The pressure section originally was supplied oil from the oil tank with a little bit of head pressure. It didn’t have to suck initially the oil was there trying to push its way through the pressure section. That’s what the stopper valve prevents when engine is off. (Head pressure is a term that means the pressure created by the vertical height of the liquid) (A Water tower is a good example of head pressure) (everyone that’s swam to the bottom of a swimming pool has experienced head pressure with their ear drums.) Any way the pressure section provides the 56.9 psig @ 4000 @ 176*f of pressurized oil to all the critical components. Then it gravity drains back to the sump where the scavenge pump returns the majority of it to the tank and supplies some of the same aerated oil to the transmission etc and then it starts all over again..at the pressure section………
