Although uncommon in air brake technology, air-over-hydraulic brakes are found every now and again.
These systems were most prevalent on 5-ton straight trucks equipped with "gas" engines. If a truck or bus is equipped with air-over-hydraulic brakes, the driver must have an air brake endorsement to operate this vehicle.
Exception: If the vehicle is NOT equipped with an air treadle (a foot valve) to operate the service brakes, the driver does not require an air brake endorsement.
These systems neither worked well nor continued with wide-spread adoption of diesel engines. In the ole' days of trucking, gas engines created a vacuum, and this air vacuum powered the air portion of the air-over-hydraulic braking systems.
Diesel engines on the other hand, do not create a vacuum and when most trucks were fitted with diesel engines in the 1970s and 80s, air-over-hydraulic brakes were no longer feasible. To operate an air brake system, diesel motors had to be fitted with an air compressor, and once fitted, a pure air brake system simply made sense.