Toyota Repair: oil pressure problems after 22R rebuild, vasoline, oil pressure

Question
I just rebuilt my 86 toyo 4X4 22R ( new head, and short block ) everything went fine except for a problem with the oil system; no prime. I found out I had to pack the pump with vasoline, then I got it to prime. The problem is, the pressure is OK at first (20psi at idle), then falls as the engine warms ( in two or three minutes it gets to 0 psi at idle) The oil pump and spline are new. All other parts from my old engine seemed in good order. What gives?

Answer
Possibly a blockage in the oil pressure passage that connects to the sending unit, verify the oil pressure reading by installing a mechanical gauge in place of the sending unit and see if the oil pressure still drops, if not replace the pressure sending unit.