Thanks for the comments. I should have added to the background that I left the
oil from the test work in the engine when I installed it, so the only priming was filling the external lines, filter, and cooler. The "out" line from the adapter plate is common with the sender and goes to the side of the filter mount, which feeds the in holes to the filter as you all have described.
I am increasingly suspecting the pressure sensor/gauge.
Mark