Engine brakes are not working isx

[Smalzfrmmt]

Ok isx masters I need your help, I work for a car hauling company as mechanic. We have an 07 379 with a Cummins Isx that burned up two pistons, and cracked the head. Truck got complete in frame new pistons new liners new head got it back together test drove tested good put the driver back in it. Driver came back 1/2 later saying jakes aren’t working. Upon further test drives we found that jakes would work while oil was cooler but once oil got to 200° f jakes wouldn’t work then when testing with valve cover off and manually turning on brake solenoids front would cut the motor but back would turn on the rockers but not cut out the motor. Does anyone have any ideas or has anyone come across this problem any help is greatly appreciated