hard stop oil pressure drops to zero

[callbald]

39 pickup had engine rebuild and now experiences pressure drops on stop. Prior to rebuild there was no pressure issue. After rebuild oil pressure ranged from 15# at idle to 30# at 40 mph. I just went back and replaced the pressure plunger, spring and cap - results pressure jumped from idle 15 to 40+ at 40 mph.

On hard stop pressure drops to zero and then jumps right back up to 15#. Thought oil pickup getting starved but oil level is good with 5 quarts. Can't figure it out.