I've seen them on some Fords from the 80's and perhaps also on IH.
While hunting I'd look at every make and model medium duty truck. Something that obviously has Dayton wheels, might have hydraulic brakes, and hopefully new enough to have disc brakes. Leave no rock, or fender, unturned.
Though Daytons are uncommon on school buses in most states or counties I expect if you do find a bus (not terribly old) with Daytons they could likely have hydraulic disc.
