...I Do NOT See what Your Problem Is!
What 'Weight' of Oil are You using?
There are NO 'High Pressure' Pumps!!
There ARE High FLOW Pumps!!
Pressure is Produced ONLY when there is Resistance to That Flow! I.E. a 'Brand-New' Re-Build with Tight Bearing Clearances...Very Thick Oil as in 50W, Clogged/Restricted Passages...Other. Last, Hydraulic Lifters, Pumping-Up, Relieving-Down, Can and Will Effect Gauge Readings.
As suggested, You may have a 'Sticking' relief Valve...Or You have a High Flow Pump Installed.
A Mechanical Gauge, Hook-Up should offer clues.
My Digital gauge will read 51 Lbs. Cold...41 Hot, (and the Engine is Well 'Broken-In', with Bearings 'Normally' 'Warn-In') Taken at the Oil Pump, while the Mechanical Gauge at the Rear of the Block reads 60+ Pounds, most of the time, and with a 'By-Pass' hose from the Pump to the 'T' Gauge Mount, in the Block Rear. I DO run a High-Flow Melidon Pump, using 10W-40 Weight Valvoline.
MJ
P.S. ...NOT 'Letting' the Crankcase 'Breathe', as in Breathers, or a PVC system, Will Drastically Effect the Efficiency of Every Pump!! ESPECIALY, When it is a Fuel Pump. pumping from a Tank with NO VENTING!! Pumps Do Not SUCK/SUCTION..."Pumps Provide/Create a VOID that is filled by 'Atmospheric Pressure". When the Atmospheric Pressure is Denied, there is NO filling of the Void, result, NO Pumping!! I'am Not saying THIS is in Your case, But to some who may read this, may realize their Problems.