Chevrolet didn't make the dunp body, so your looking for the big truck service manual that includes the C-50 Series. It may cover C-40 thru 100 Series trucks. You could also look on eBay for a Motor's Truck Manual from the mid 60's, they can be a good resource as well.
The "air thing" is called a "Hydrovac" and it's a vacuum power brake booster. They are still available rebuilt from NAPA.
If the shoes are to far away from the drum you won't have enough fluid in the master cylindar to make contact with just one push on the pedal. My 1957 Fire Engine has an ecentric bolt with a nut on the back of the backing plate located at 3 and 9 o'clock. When I rotate the nut if moves the shoes closer to the drum. Adjust them so there is a little bit of drag with the tire off the ground and your spinning the wheel by hand.
Good Luck!
Mike B
