360 Oil Pressure

Hi all I'm new to this post and have a question I was hoping to get some help with. Last night I finally got my 360 to crank. It probably ran for 20 seconds +/- 5 secs. However in that short time my oil pressure never jumped up. I took the distributor off and reprimed it, but never had a chance to try and start it again last night to see if it would keep running and build pressure. When I primed the motor with a drill it probably took 30 seconds or better to start building pressure and went to 70 psi and was still climbing before I stopped. Is this a normal amount of time for the AMC motor to build pressure or do I have a oil pump problem? If this is not normal what do I need to check or do? Please Help!