buse safety requires abv's fail safe brakes, page-2

sounds more like just a lack of maintenance or operator error on the current Merc bus brakes.
I had an air brake hand break lever on my Isuzu truck, its either fully on or off as the report quoted, thing with large trucks and buses is the calipers are kept out by the hydraulics and when the hydraulics fail the brakes automaticly lock on so fail safe
Compare that with a landcruiser or any light vehicle where if the handbrake is loosened or hydraulic pressure is lost the calipres release from the drum or disc.