It could be done but air brakes don't work like conversional brakes were the braking force pushes the pads in, they work on a different system were the pads are always trying lock onto the rim but the air pistons actually hold them apart. The main reason this system was developed was for lorrys so that if there is a brake faliure the brakes lock on instead of having no brakes at all. Like I said it could be done but not really any need for it although for some one intrestead in enginering i'd like to see it done.