I am not adding air bags, I am simply replacing one that is close to 28 yrs old and another that is somewhat newer that are already on the back/RV portion of my rig.
It appears everything on the truck was well thought out/shop built....after 28 yrs, mostly parked, things need maintenance.
As a side note,
http://www.fsip.com has a complete listing of all Firestone air bags, complete with PDFs and dimensions.