allison md3060 and non-functioning exhaust brake

So, tell us how far into the "exhaust brake cycle" you get when this failure occurs:

1. Does the Allison shift pad left window go from "6" to your exhaust brake pre-select gear (can be anything, but "2" and "4" are most common)?

2. Does the transmission start downshifting toward the pre-select gear?

If both of the above happen, my next move would be to temporarily wire a voltmeter or even test light to the exhaust brake solenoid to see if it is being activated.