You should have a toggle switch on the dash. Have you got power to that?
The exhaust break on the exhaust manifold requires lubrication on the pivot points. See your owners manual.
The exhaust break can get a carbon buildup and stick if not used much, or used continuously at lower engine temperatures. This is hard to fix without removing it. Like anything else, it is designed to operate at engine working temp. that generally will keep carbon buildup under control. If you can get it working just try to cycle it to help free it up.
ps You have to be on the road for it to work as it is contolled through the Allison. I have not had the problem as I am in mountain country and my exhaust brake gets plenty of use.
"LOVE and LOSS, are two of the greatest emotions one can experience. -- I went to school to learn about "WHAT GOES UP MUST COME DOWN" but I had to live my life to learn the lesson of: 'WITH LOVE THERE WILL BE SORROW'."
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