I"ve missed something or I'm confused because I can't seem to connect closing the drain petcock with solving an oil
pressure problem. Wait a second, is it possible the drain system is tee'd with the oil pickup tube? That is a scary thought with disaster writen all over it. If it were my engine
I'd keep searching till the interaction between the drain petcock and oil pump was completely understood, and possibly eliminated. I'm very pleased to hear you have oil pressure again though.