Air brakes are slower and FAR jerkier than hydraulic brakes.Even 85 is pushing it. Fully loaded we are sitting around 18000+ lbs. That's a lot of weight going really fast, with disc brakes.
I always wondered why Type I and III units don't use air brakes but I guess it would be a total pain in the butt to get every employee a CDL with an airbrake endorsement and have it as a prereq for employment.