If the pump is checking out as good,it should build the same pressure 3100 psi ,assuming the relief valves are set the same,when hooked up to either the 6 spool section,or 3 spool section.Id hook it to the 6 spool,check psi,then swap reliefs again and check psi.If everything is pretty close on the psi,and its not going past 1500 psi on the 3 spool valve,there has to be some internal leakage in that section of valves.