If the bypass checks out OK, pull the engine/oil pan and check baffle spotwelded in the oil pan, baffle breaks out and lays flat in the oil pan. At Idle works OK, HI rpm the suction tube will pickup baffle in oil pan and cutoff/restrict oil flow. Have personally seen this in more than one 1400 series.
If psi has been at near zero at any point you will need to freshen up the bottom end now as brgs are likely damaged due to low oil and could spin one sooner than later.