For what It's worth, I don't believe that GM ever offered an air over hydraulic brake from the factory. We had vacuum assist hydraulic as standard on the Chevrolet trucks and, later, full air brakes.
We did a dealer install of the air over hydraulic brakes primarily when we were setting up a tractor so we'd be able to actuate the full air brakes on the trailer. I had one customer that bought a new tractor from us about every three years that hauled cattle from Greeley to Denver two or three times a week. They weren't very heavy trucks, but they worked OK. The air over hydraulic brake could apply more pressure the the hydraulic system and you could lock up the wheels.
