isx15 jake problems

OK, does anyone have any experience with diagnosing problems with cummins jakes? the motor is an isx15 cm871 485hp 1850tq. the issue i am having is the jake doesn't hold back as well as it should. sometimes I'll know it is functioning properly and it'll build 30-35 psi manifold pressure, but most of the time it only makes about 20 psi. i had it to peterbilt any they found one of the jake heads was not coming on, diagnosed the ecm as the problem, but no change. i brought it back to them and they found an air leak on the pressure side of the turbo and some loose wiring. after that it seemed to work well for a day and then back to the same intermittent problems, although more often than not it doesn't work. no fault codes. hope someone can help me, I've only had the truck about 2 weeks and it's killing me. I am done with the dealer and their no answers, although I don't really have any time to be messing with this I need to learn these isx motors for myself.