Jake problem

Have a 2000 w9 430 detroit. For first few miles and minutes of running, all 3 stages of jakes work properly. But after few miles down road it almost loses the third stage. I can select back and forth 2 and 3 stages and it makes very slight noise change as well as about 2psi boost difference. Back in April, had a injector termial to break and shop replaced that and one jake they said had a broken bolt/spring. (can't remember exactly as i don't have reciept in front of me now but was $400 part) Picked truck up and did perfect for a while now it's back to same as before.
I pulled valve cover off and can see new jake but do not know what bolt/spring could be broken. Is there common part that breaks in jakes or maybe a wiring issue??? Makes no sense it works when first going then quits. Truck has smart wheel too. Truck is also not driven much