I had a CDL, but I was grandfathered in when they introduced them (I can already hear the jokes). I don't see how it could be that difficult. I have no idea what they could ask about air brakes, other than you have to be sure you have pressure and no leaks. maybe that they have a little more lag time before they take hold... don't know.
When it expired I was in a different job and didn't want anything to do with driving, so I let it expire. I hope you aren't thinking of driving over the road, it sucks big time. I did it for 19 years and it was the best thing I ever did for myself to quit.