1985 Mitsubishi L200 NO engine oil pressure

Engine Mechanical problem
1985 Mitsubishi L200 4 cyl Four Wheel Drive Manual 67000k miles

Engine has lost oil pressure. Was driven for 20km, stopped, then when restarted had no oil pressure at all. Engine was rebuilt 67000km prior. Have 15 years history of vehicle. Oil & filter changed 100km prior. Strip down & full inspection of oil pump, pickup, filter, bearings, releif valve, gaskets, has revealed no reason for loss of oil pressure. The internals are as clean as a new car off the production line. What am I missing? What else could cause this situation? We are going to reassemble with new oil pump, releif valve & spring, even though they all appear in good working order. Any ideas would be appreciated.