had this once with my xr4x4 caused by sitting in traffic for an hour with my foot on the brakes....DOH!!! pedal went rock solid and took serious effort to stop allthough luckily after i got home (and felt the pump and servo which were damn near glowing lol!) after letting it cool down all was fine. this probably wont help you with your fault but may help other sierra abs owners from doing the same dim witted thing i did lol!