Truck Repair: 1988 or 89 855 Cummins big cam, jake brakes, cam engine

Question
Hi ... My brother has a dumptruck with a 1988 or 89 Cummins 855 big cam engine that has suddenly developed an oil pressure issue after the engine heats up (after 10+ or s miles that also causes his jake brakes to cease working. When he starts her up the oil pressure is steady at 21# and the jakes function ... AFTER driving and the engine heating up to normal range the oil pressure drops to about 10# and the jakes no longer function. He has changed almost everything short of the oil pump, including gaskets. Nothing having to do with the jakes at this point. The oil level remains full.
This engine was rebuilt in frame 150,000 miles ago. Due to her age, she is all mechanical with no computer for diagnostic codes.
This is getting quite frustrating and we are looking forward to your help with this.
Thank You for any assistance.
Jodi

Answer
Jodi,
I need to clarify the oil pressure value you quoted is it the same as 10 p.s.i and 21 p.s.i. If so, I believe these values are too low for an 855 big cam.
The fact that the oil pressure falls to 10 p.s.i as the oil thins out indicate a problem with your oil pump. Please effect a replecement or repair as necessary.

Kind regards,