I have had the same problem with breaking bolts. I have the Firestone bags and I think what happened is the bolt holding the bracket to the overload spring broke and then the entire lower bracket bent. I had to buy a new set of airbags with brackets. I replaced the bolts with grade 8. Used a torque wrench to tighten everything and have still had a bolt break.
I now have stableloads upper and lower and run about 30 PSI in airbags.
So far no more bolt breakage, but I check after every trip. Hopefully it lasts. I now carry spare bolts with me.