This same issue happened to me today. Our M923A2 has been working perfectly until today. I started it up, waited until the air pressure built up, and then released the parking brake, moved the trans shifter from neutral to 5-1. The truck would not move forward. I could move in reverse, but it was obvious that the brakes were on. I called a fellow friend and he suggested pushing in the override knob. It worked and allowed me to move the truck normally in forward and reverse. Of course the light was on that indicated a parking brake problem. I moved the truck to it's normal parking place, then pulled out the knob to normal location, which locked up the brakes. It was getting dark by now, so I will check it out tomorrow after reading more on the TM's. I did notice that the roller that lifts when you pull the parking brake up would move up and down with ease.
I am glad I found this thread.