Jake’s always applied with brake pedal

[Brookhaven66]

Looked at a friends Recently purchased 2001 diesel pusher motor home ism500 Cummins
Spartan/Freightliner chassis,Allison auto trans the other day.
Jake brakes work as should going down the road, the low/high setting seems to work and on/off switch seems to turn Jake’s off. Accept that “anytime” you depress the brake pedal it appears the Jake’s engage, even at idle with parking brake on or off, step on the brake and engine will pop and bang like it’s going to stall, but clears up when you get off the brake. Have to unplug the brake light fuse to remedy this issue or unplugged both air operated brake switches under front windshield coming off of brake pedal valve assembly,either of which disables the coach’s brake lights. No codes on Insite, no Wabco codes, no Allison transmission codes. Also don’t see any data from any module that would lead in any direction. Also this will happen with or without the Jake switch turned in off or on position. The Jake will disengage going down the highway when you flip switch to off position,so I assume the ecm sees the command, but on insite at stop with just key on or even engine running, the insite always shows switch on. I can’t see any other body or chassis modules to communicate with from my nexiq2 data test in 1939 or 1708,
Any advice much appreciated