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.

Jon