Is the distributor seated and driving the oil pump?

The pump has an internal pressure relief valve, small chance the spring is broken or trash stuck in the seat holding it open.

You should have pressure from that port for the gauge with engine running.

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-