Air over hydraulic is one of those things that had its day, but that day is long gone. I am assuming that you actually use the truck for something. If it were mine, I would look for a steering axle with air brakes and go to straight air. As far as air disc brakes, different configurations have been rolled out over the past 30 years as a new revolution in braking but they have never caught on. In fact I have bought several trucks that had disc brakes on them for well below the market. I converted them to drum brakes to use them.