WHILE FOLLOWING OTHER NORMALLY TRAVELING VEHICLES DOWNHILL - VEHICLES IN FRONT CAME TO A STOP - BRAKE PEDDLE WAS FULLY DEPRESSED...NO RESPONSE...BRAKE FULLY APPLIED 2ND TIME...NO RESPONSE...RESULTING IN A REAR END COLLISION MULTIPLE VEHICLES...POSSIBLE TOTAL OF MY 2007 SIERRA. DAMAGES ARE VERY EXTENSIVE . INSURANCE TO INVESTIGATE GM BRAKE POSSIBLE CAUSE....AIRBAG DEPLOYED CHEST & NECK INJURIES....MANY OTHER INCIDENTS INVOLVING GMC BRAKE SYSTEMS. NEED IMMEDIATE NHTSA INVESTIGATIONS !! THIS COULD HAVE CAUSED A FATALITY. *TR