I beleive I've got a 1974 SD455. Engine oil pressure has been tapering off the last few times I had been driving it, untill the last ride, Oil pressure was sitting @ 20, would not rise with engine rpm. Not common for this car typical 40-50 standard warm oil pressure.
Yanked motor, checked oil pan, no metal/bearings in oil, thought pump went bad... replaced oil pump reinstalled engine. Started back up, engine idle oil pressure around 40, let IDLED for 5 or 10 min, rapped throttle once to aprox 3000rpm..... oil pressure back down to zero!?!?
Had talked to a an engine builder in my area, he suggested that the oil pressure bypass could be to blame??

Any thoughts on what you've herd so far?? I'm about to take motor out again??? Would like your thoughts!!

Please and thank you

Glen
Quicknine@aol.com      

A guy that I know had one of this era with simialar problem when under warranty. Dealer had a factory service bulletin on it and able to fix the problem. I can't recall the fix but will call him to see if he can give us a clue. It was something simple like the relief spring.  

Just to be sure, you did try another guage right? How about the lifter gallery plugs behind the cam gear? The bypass relief spring and valve could be it as well.  

that sounds familiar i have a pontiac 69 350 that had  20pounds at idle and rose with engine rpm but not what i wanted since i was working the engine a little. i replaced it with a new one and for about a week i had great pressure 35 at idle and about 70 under throttle. then suddenly it dropped to about 20 and would rise with engine rpm but now my gauge just reads zero when warm when cold its at 100pounds. but warm its just at zero. never fixed the problem (new gauge, new lines, checked EVERYTHING) pulled my valve covers with engine idling and there was plenty of oil sloshing around so now its been a year with this "zero" oil pressure finally i just pulled the gauge. the engine runs strong and i know its got oil pressure so i just gave up on it. noone could figure it out so if anyone has any ideas on how i could possibly fix this that would be great its been a year!