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Megahurtz has a good point about the low side check valve possibly being a choke point.
But... if it were, then you would see low side pressure being sucked down from the high pressure pump pulling from the line till it became a deficiency. It wouldn't stay even close to normal if it were losing enough volume to be 2000psi low.
And like i has said earlier. If the hpfp return valve is stuck open then you're going to see things like 200psi on the low side since it vents back into that line.
If the cam and follower aren't worn down then to me it seems like a high pressure pump.
But I've seen so many high pressure sensors fail on GM that i wouldn't rule that out. If you have the ability to check pressure with a gauge you would be able to determine that. If not maybe just throw one at it for good measure.
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