Your tractor protection valve has a quick release valve in it that might be hung. The tractor protection valve has a small air line from the lever on the dash. It also has 4 larger air lines, 2 of which go to the trailer. Find the valve, and have someone flip the toggle switch and see if air comes out of the valve. If its bad, it doesn't necessarily have to be GM. As far as I know, all trucks with the toggle type switch used the same tractor protection valve.
