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I rebuilt my 5.3 and it has low oil pressure. I replaced everything. For starters I replaced the oil pump twice thinking it had a bad pump no change. Replaced the oring still no change. All the bearings look excellent. The cam bearings are lined up. Mechanical gauge reads 25 cold and 7 hot. The barbell and front plug are in place. I checked the front plug behind the alternator bracket with the gauge and it reads 60 cold and 27 hot idling. Then went to the back of the block above the oil filter and checked the pressure there. 25 cold and 7 hot idling. Is there a chance the oil pan could be causing this? I changed the oil and filter and no change. I use the acdelco filter. I did change the pan cause the old pan was cracked. I got the new pan from a junkyard. I want to say it's the pan cause the pressure drop is before the bearings and cam and crank. Thanks for your help.
Possible the o-ring on the barbell got nicked? If it's compromised it'll let the pressure bleed back to the rear main seal housing. May explain why there's pressure in the front, but not in the rear.
Assuming the plug is in the front of the block, it sounds like an issue with the barbell in the back of the block. This is what diverts the oil through the pan to the filter and back up through the block.
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Rebuilt 5.3 low oil pressure 
