As I understand it, this is a graph of the bag as used on the GMC WITH the cones, which are called PISTONS by Firstone.

 

Left side scale is pounds of load supported.
Bottom scale is inches in length/extension of the bag.
The curved lines are the air pressure in PSI.
This chart is dated 3-6-73, and the guy that helped find it said he had to really dig the archives for it.