Air bags can deflate through leakage:
1. the bag has become porous from age, or otherwise damaged
2. the sealing surface where it sits in the cradle has gone, again possibly age related
3. a faulty valve allowing air to bleed out
If the rear is going dowen in less than around 6 hours, it would be advisable to get them checked.
The Sydney Classic and Antique Truck Show
On again 16 June 2013
@ The Museum of Fire.