Just another source of info on these pumps from Melling:
Technical - Melling
The first entry in their section on "
Common Misconceptions on Oil Systems" (with the answer following the colon) is:
Oil pumps produce pressure: Oil pumps produce flow and the resistance to that flow produces pressure. The pressure relief valve limits the maximum pressure but does not do anything until the pressure has reached that point.
(Emphasis is mine...)
They did make one editorial mistake on that page.... see if you can spot it.