I'm new to Club Hotrod so I hope I don't bore anyone with something you have answer before.

I have a 454. After starting I have 55 PSI oil pressure. After warming up at about 2500 RPM I have 10-20 PSI( in 4th gear at 55 mph). At Idle it is for all practical purposes zero( there is oil coming up out of the pushrods however).I just put a new Comp roller cam in and oil pump as the last cam got wiped out on 3 lobes. I bought the car with this problem and didn't know it! The engime presumably had 5000 miles on it when I bought it and when I had the motor apart it was very clean inside with the exception of the cam lobes. In the meantime, there are a couple of lifters if I push down on the push rod compresses so it seems the mechnail lifers are not pumping up? This all seems to be leading to pull the motor out again of which I'm trying to avoid. Any ideas how I can identify the root cause with out pulling the motor apart again? It seems the oil pressure was the same way before I put the cam in.