I recently rebuilt my rams 5.9l 360 engine with an engine re main kit. After dropping the motor back into the truck and starting I lost all oil pressure when it warmed up. I replaced the oil pump, pickup tube, and pressure sending unit. After attaching a manual gauge I still had no pressure. In fact I steadily lost pressure as the temp rose. What could be causing this? I have heard all kinds of excuses for it but I need to know for sure so that I can get my truck on the road. Thank you for any information you can provide.