D/B big 4 no oil pressure

Hello. First post on this site- just not getting responses like I used to on " the other" . Open drive D/B big 4,compressor. Customer wanted hemorging service valves replaced. After upon start up and initial charge comp times out and trips oil safety- happens to have spare oil pump- replaced no change- still no oil- pressure and no oil movement in site glass. Came back with a fresh head- realized motor was turning opposite of neighboring comp- also realized I put the oil pump in wrong - it was about 2 am after going 18 plus hours so give me a break-corrected rotation and oil pump installation and wooosh magic oil pressure and movement worked on charging system back up ran for about 45 minutes then bam- same thing no oil pressure no oil movement in site glass. Put a tee on oil circuit port on comp to check oil circuit prime- just spits and gurgles at suction pressure around 15-20 psi is 437A system. Can't understand why it work for 45 minutes then just stop- how do we prevent oil pump from vapor locking , and does anyone know where I can get a exploded view of compressor/IOM manual so I'm not going in blind as i think I need to take end bell off to check oil strainer. DB "tech support" no help. Thanks

Hi Dave
Might be a silly question but is the system flooding back, this could be causing the loss of oil pressure after 45 mins. Check the internal oil stariner by all means but the problem may not be the compressor.
Regards
Paul

No flooding as system would have been slightly undercharged as I still had 100 plus pounds to go what I can't got vapor out of oil tee and crew said it had happens before and they had to bleed the pressure of the comp to rectify the situation- I tried the same and evacuated the isolated comp with no change on start up