I had a similar event when, I built a 350 Vette engine. Spoke to the people at GM-Corvette Div. I new and they told me to drop the pan remove the pump and take out the spring and ball check and reassemble, rust sometimes stops the ball from moving, it solved the problem. The problem is what happened to me was the pressure was high enough that it blew the
oil filter seal and in seconds dumped 2-3 quarts of
oil all over. You certainly don't want that to happen the first time you take yours out on the road.
Bill K