i would guess that your oil clearances are too high. this creates too large of a "leak" and lowers oil pressure. do you know if the clearances were checked when the bearings were replaced? usually with a bad pressure releif valve, you get no oil pressure. oil pressure is created when the volume of the pump is more that the "leaks" in the engine. all of the "leaks" are enginered to be a certain amount if you have a tolerance that is too high, then you get low oil pressure.