If it has good pressure initially, and after running for a period of time the pressure drops to zero, its because the pump doesn't have any oil to pump---all your oil is probably upstairs-------------

A thing to try---time how long it takes for the pressure to fall and shut it off, wait 10 15 minutes and try again---same thing and time? now add 2 or 3 qts and see if it takes longer time---------If you want to be exact and truly scientific use 4 extra qts (1 in filter) and the time should double-----Quite possible the Canfield heads don't have the drainback holes drilled or they are in wrong place------------also look for casting flash/junk in the drains in the valley--------