And it loses prime.
Can't get oil pressure started.
Ran it for about 15 seconds and nothing.
Had to put the priming shaft into the distributor hole and use the drill.
Pressure came right up.
Started right up with 72psi.
6 days later, same thing.
This never happened before.
What causes this to happen?And how do you fix it.
Dual milodon oil lines. It's the new external Black oil pump.

Did you loop the suction lines above the pump, so that there always is some oil trapped and can´t go back to the pan?

Michael

if you slow the drill speed down to as best you can match the "cranking" RPM on the intergear will it prime? I'm wondering if it is bleeding off somewhere from a leak or not priming in the first place, just thinking out loud. Post what it ends up being.

6 seconds on the drill and it's got oil pressure.

wix/napa filter?

It is loosing the seal between the pump and the block or somewhere else allowing the oil in the lines to drain down to the oil pan level. Look very closely at the O rings and oil line fittings and maybe even the lines to fix any problems with them
Are you using the stock type oil pump gasket to the block? If so get rid of that That pump only needs the O rings on the pump body and snout to seal properly

Not this build.
Morosos

I've had 1 small block Melling pumps pressure by-pass piston stick open and have no pressure when cold. Also an old 2.2's pump did the same.
Big block would be easy to check that piston.

X2, a pin hole will break suction.

You have an internal air leak in the pickup system. Very common problem with the old Milodon dual external setup. It is less common problem with an internal pickup but can still happen.

It's the new Milodon oil pump and cover setup.

If it has an external line then that is usually the problem. If it is all internal then you probably have an issue with the pickup tube. Maybe a small crack in the tube or the block is cracked in the threads or something like that. Internal problems are rare but problems with the external lines are very, very common. I stopped using the external line systems because they were such a hassle.

When did this all start ?

You make no mention of that.
Fresh engine build, or been on the road untouched for years or something in between?

read title ,said to myself ..bet its got "Dual milodon oil lines"

Mark

When you have an air leak into the pump, the “loop” makes the situation worse. You now have longer hoses filled with more air than you would with “normal” hoses.
A siphon is a siphon. Stop the air leak and you will lose the siphon.
I went through a season of this with my car. Never an external leak. I had the older cast 2 piece milodon pump. At seasons end, replaced the pump gearset, all the orings, resurfaced the pump cover, and had it hard anodized. (There were some minor radial scratches in the cover)
Never found a clear failed part, but doing this stopped the issue. My suspicion has been that the pump clearances, along with cover scratches, allowed air from the outlet to leak into the inlet. Just my best guess.

Dual external -12AN lines

Take it all apart, clean and inspect everything. Put it back together with plenty of lube and torque everything to the proper specs. If that doesn't fix it then you'll have to try it again until you find the air leak.

the dual swinging pick-ups have issues sometimes. I always used the static dragster pick-ups. that being said i'll be the odd man out here. try a different filter first before dismantling the mechanicals. I've had trouble with drain back on every wix, napa, k&n, and I hate to say but baldwins too. baldwins used to be top notch. the anti-drainback in the wix, etc, etc, doesn't work when the filter is in the horizontal. people will argue this but I doubt most people know or understand drain back. currently i'm using purolator pl30001's without issue. other brands may work too. a $10 test is where i'd go first.

That could work. I use the small Mopar 409 filters on my stuff these days. The little filters fill up with oil faster and they are stronger since they are smaller. Screw one of those on there and see if it solves the problem.