I'm not sure I agree with Pete on this one, but there is a test of the Jake brake while parked that I have used in the past.

A Jake is essentially a system that turns your engine from a power unit to an air compressor. When it functions as an air compressor, it has the compression stroke the engine would have when used for power, but at the end of the compression stroke, the Jake mechanisms open the exhaust valves so none of the work of compression is used to push the piston back down. That exhausted compressed air is the noise heard when the Jake functions.

What makes it work is a mechanism located in the cylinder head under the valve covers. There are two solenoid valves in each head. They have to work for the Jake to work. Your problem could be a failed switch, failed solenoid valves, broken wires so to determine if the solenoids are functioning look in the large rear electric box. I don't know the exact post number, but it is likely to be listed on the diagram for the electric terminals and it will be described as engine brake or retarder or something similar.

Do not go by my memory, but the post numbers for the Jake are going to be low, single digit, like posts #1 and #2, or #2 and #3. They varied slightly from coach to coach as Prevost made design changes. With the engine idling (be careful around the belts) use a jumper from the 12V post (on the bottom of the box, check with the voltmeter) to each of the Jake posts in the box. Leave the jumper on for a few seconds and if the solenoids are functioning properly you will hear the engine lug down as if it were under load.

Check each post by using the jumper. If only one post lugs down the engine focus on the side that did not lug to insure you have continuity all the way to the solenoids. You may have to remove the valve cover. If both sides lug the engine down then you can rule out any further investigation relating to the solenoids or the wires going under the valve cover to them and should instead focus from the switch at the driver's seat back.

Hope this helps.