This has been a known problem since the '70's and has nothing to do with sealing up. If sealing were the issue then you'd be drawing air into the pump with the oil - definately NOT GOOD!

On a stock pump the feed line comes up through the block and down into the pump. No matter what, it will hold some prime.

However, with the Milodon pump setup either the single or dual lines both feed up and into the pump. When the engine is off, the oil just runs back to the pan.

Of course in order to do this it needs to have air bleeding in from somewhere - like around lifters, cam bearings, rockers, etc.. And any oil below the pump will bleed off - like around the crank.

The bleeding off speed will depend on the thickness of oil and how much your clearances are.

The solution was mentioned ... run the lines so a section is above the pump. Another solution would be a -12 check valve in each line (Summit - not cheap!)