Trevor, congrats and good luck with your new toy.
Here's the place to go for Jake brake info - http://www.jakebrake.com/support/
They normally won't activate at idle speed,like BK has already said.
The basic fast idle circuit is: a electrically controlled (from dash switch and a relay) solenoid valve allowing air pressure to travel to a mechanical cylinder, mounted on governor cover of your engine, moving internal governor linkage to raise idle speed.The parking brake must be applied, there is a switch plumbed into the park brake air circuit for this too.
I suggest trying to get a copy of a service manual for your bus. It will have TONS of useful info and electrical diagrams too.Send e-mail or catch me in chat room, I'll help you as best as I can.