Does it crack in the weldment itself, or beside the weld? I just went out and looked at my bus, which is the same, the lower radius arm is basically captured on a tube that extends through the post from side to side. The uppers I couldn't see clearly. If it broke it's going to have been simple fatigue, most likely, all front end braking loads go through those posts, as well as some suspension control loads and steering loads, but not much of those two. If it cracked again, put it down to a faulty weld with insufficient penetration or HAZ control. It's probably just plain mild steel, from the rust I see on mine. Once something under cyclic loads cracks it's actually quite hard to repair without replacing so I wouldn't feel bad about your welding
. Also - where exactly is your crack located?