barry is right on the money about the bent and broken back frame situation. in every case i have seen with the "gangsta' lean" [i'm a poet 'n don't know it ! ] , it was a bent or broken frame, or both. several times i have been able to fix them by skinning back a small section of the cover for the repair, but mostly, the cover has to come off.
also, if the frame is bent, it's a good idea [in my opinion] to reinforce the bent area to prevent it from bending again, forcing you to repeat the repair. after all, the assembly is 50 years old and has been through countless cycles in it's lifetime. the other option is to acquire another seat frame.