It bid to 30,750
Why would you put this air pac on in place of the vacume hydro vac, I built the same truck for Dennis and it'll put you through the windshield.
AIr-pac's like Tony posted, were used in post war years to allow hyd braked trucks to pull air braked trailers.
Air over hyd master cyl were used for a few different reasons. Ford used to for disk front brakes, when air disk calipers were in their infancy, The Brit buses and I believe Mack used it when there wasn't room around the wheel end for S cams or wedge brakes. The hyd wheel cyl could fit entirely within the brake drum.
To fit an air-pac to a (formally) Hydovac truck, all you had to do is replace the hydrovac with the air-pac and add a compressor and air tank. The rest of the hyd brake system remained as designed.
I have three of the hydrivac's and they work very well. Yeah they can faceplant you if you stand on them. The air pac was a little smaller (using higher pressure then atmospheric) but the basic design is the same, foot pedal controls how much "boost" gets applied at the booster, since the booster uses air pressure, it creates a modulated air signal based on how much braking effort is applied at the pedal, exactly what is needed for an air braked trailer "blue line" the red line gets air system pressure.
If you have a good understanding of hydrovacs, air-pac are almost identical.
Air over hyd is an air system, with a typical treadle valve up to the delivery to the wheel end, instead of S cams or wedge, the air pressure operates a std hyd master cyl that is then fed to hdy wheel cyl. More complex, in that it has to have all the air stuff before the master that a full air braked truck would and doesn't have the back-up that a hydrovac or air-pac does. Both of those systems will operate "un boosted" if they loose air or vacuum, a Air over hyd will have no brakes without air pressure.