Rebuilt Engine, Super Low Oil Pressure :(

Had this happen in the 1980's. Just rebuilt, installed, started and had good pressure, drove home, parked and started to go to the store, had maybe 30 miles on it and pressure dropped to 7 psi. Oil pump was bad. Something about the pressure check spring stuck. Changed the pump, in the car, and was good to go thru 3 different cars and is still waiting on another.