We had something similar happen on a B engine that sat for a long time. Sitting for 15 years, install it in our 69 Charger, got it running, changed the oil, i can’t recall but seems like we had driven it 20 miles or so, everything was working fine. The next time we started there was no oil pressure. We pulled the oil pump off and inspected it and could not find anything wrong. Before it was put back on we filled the pump with oil to prime it. Then reinstalled it. Immediately it had normal oil pressure and never had a issue after that. The pump had lost it’s prime. We didn’t stop quickly or anything, I think it must have had some air in the filter. Kept a pretty close eye on the gauge for several months after that happened, then eventually forgot and treated it like any other engine.
Since it sound like a new build, with fresh oil, I would guess the trouble was a combination of some air in the filter yet and the pick up getting uncovered for a very very short time from braking hard. The pump lost its prime. If all the air was out of the system (likely just in the filter) it probably wouldn’t have lost its prime. It might be more likely to happen with lighter weight oils, or oils that pour easily. I think we had a cheaper 10w30 or 15w40 in the engine that had the issue. It had about 80K miles on it at the time.
