Exhaust brake not working

Probably need some clarification.

Cummins 400 HP generally means the Cummins ISL. The ISL could be ordered with either exhaust brake or engine compression brake (for more $$).

If Cummins ISC, it will have an exhaust brake.

Let's see how far into "engine brake is on" you get: Does the Allison shift pad change from "6" to the lower gear programmed as the engine brake pre-select?

What brand of exhaust brake do you have. If PacBrake, has it been lubricated:https://pacbrake.com/product/c18037-superlube/

And, very easy to verify if any type of engine brake is working: Use transmission down arrow to select a gear where you are close to the upper end of the RPM range. Turn engine brake on/off/on. Since the transmission is now a constant, easy to see if you have added braking HP.

Curious to know how you determined that exhaust brake isn't working? If the brake in your coach is activated with the dash switch, it will close automatically when you begin to slow down. Typically, this will also cause your transmission to downshift to a lower gear, (typically to 3rd) The actuator for the brake is an electrically operated solenoid that pulls the brake valve closed. I believe you also have a Cummins ISC engine.

The problem with determining it is not working, is that it only activates when you are slowing down, and then you have to visually see the solenoid action. That is not an easy task. To see mine work, I have to raise the mattress, and open a hatch while the coach is driving. This requires a second person to watch while I drive.

You can test the operation of the valve by applying 12VDC to the solenoid wires, while not moving, but that doesn't guarantee that the 12VDC will be applied when driving.

You also have to periodically lubricate the valve mechanism which special exhaust brake lubricant. This will insure that valve mechanism is free to move. They are known to get stuck from rust. You can test the mechanical operation with your hand by turning the operating mechanism to verify it opens and springs closed when you let go.

Could be many things...first I would check the switch on the foot brake pedal...on 18 wheeler trucks that I drive dirt/water can get in the switch and cause it not to work.