I have the Firestone bags in the front suspension of my 86 Dodge snowplow truck. They take a real licking and have serious rust on the mounting plates, but neither one has ever gone flat. Due to the rust problem, the left one has a slow leak, dropping about 15 psi/day. With the contractor size (& weight) blade, plus the weight of the snow/ice hanging on the blade, I only need a max of 45 psi. So I air up before doing my 30 plowing accounts and never have to worry about it after that.
The front bags have separate air supply lines to eliminate the progressive lean of the truck at highway speed going around a curve. Rear bags need to be isolated for the same reason, but may be even more of a problem with the high hitch weight and high center of gravity of our 5ers.
Paul_in_MN