I've seen two different styles of AOHB systems (there may be more). The first and most common is to have a normal hydraulic brake system with the Air Booster Teed into the system just like the Hydrovac system. The AOHB system does require a air compressor and air tanks.
The second type is a system that looks like a Full Air setup with a treadle pedal and two air chambers (one for each axle) with Master Cylinders connected to then mounted on the frame rail. The down side to this system is if you loose air, you have NO brakes as there is no mechanical/hydraulic connection between the pedal and the drums like there is in the first example.
If I ever restore my '56 9200 LCF that has the second type system I will convert it to Full Air with rear Maxi brakes.
I've driven antique trucks with Hydrovac's for 25 years (40K miles or more) and never needed to service them other than a shot of light weight oil in the can every now and then to keep the leather seal soft.
Mike B
