I don't yet have manuals on the Genesis to know all parts. That being said, my bus does have bendix parts.
This is the rear air brake can with the spring brake (for parking brake).
The shaft coming out of the can connects to the slack adjuster. Bendix is a major supplier of brake parts for all different kinds of busses, trucks, ect...
You can't really check the slack on the rear unless you have sufficient air in the system, block wheels, release brakes. Kind of sketchy by yourself! With no air in the system, or low air, the spring brakes automatically apply. Since you are buying from a mechanic he should be able to walk you through it without much trouble.
It's cool all the fluids and filters are new! The school district where I got mine had just done a complete service also. Nice for peace of mind on the drive home.