help needed

Have recently rebuilt a perkins 6354 in an old massey fergusson 400 dozzer all linners bearings rings etc where changed . I also changed the oil pump and main relief valve and on start up I am only getting around 15 psi oil pressure . There is no external oil leaks I believe the oil is bypassing internally but do not no where (the new oil pump has got a drain channel machined in housing near drive spline where the old one does not could this explain my low oil pressure) .Any ideas would be greatly appreciated

Has it got the correct oil filter on it now?
I have replaced 2 perkins engines in accos.
Both were starved of oil and ruined the crankshafts.
(incorrect filters, via a wrong cross reference)
We also had one, for whatever reason, could not initially pick up the oil.
We pressure fed it intially and it has been fine ever since

chocs