2013 GMC Sierra Brakes Problems

I OWN A 2013 GMC SIERRA 4WD CREW CAB THAT IS HAVING BRAKING PROBLEMS. I PURCHASED THE VEHICLE ON 05/29/2013 NEW FROM BILL SMITH BUICK-GMC INC. LOCATED IN CULLMAN, AL AS A NEW VEHICLE AND AT THE TIME OF PURCHASE IT HAD 7173 MILES ON IT AS STATED ON THE ODOMETER DISCLOSURE STATEMENT. THE VEHICLE CURRENTLY HAS 41585 MILES ON IT THAT INDICATED I HAVE DRIVEN THE VEHICLE 36412 MILES. I HAVE HAD TWO INCIDENTS WITH THE BRAKES WHERE THE VEHICLE FAILED TO STOP WITH THE BRAKES FULLY APPLIED. THE FIRST TIME WAS WHEN THE VEHICLE HAD APPROXIMATELY 15000 MILES ON IT. THE SECOND TIME OCCURRED THIS PAST THURSDAY, DECEMBER 18, 2014 WHEN I APPLIED THE BRAKES AT 25 MPH AND THE CAR JUST CONTINUED TO ROLL UP HILL UNTIL I HIT A CAR IN FRONT OF ME. THERE WAS NO DAMAGE TO THE OLDER BUICK LASABER REAR END OR BUMPER BUT MY FRONT BUMPER WAS PUSHED IN ABOUT 2 INCHES. IF I HAD BEEN ON THE LEVEL OR WAS GOING ABOVE 25 MPH I SUSPECT THERE WOULD HAVE BEEN SUBSTANTIAL DAMAGE TO BOTH VEHICLES. WHAT?S HAPPENING IS WHEN THE BRAKE IS APPLIED THE CAR STARTS TO SHUDDER AND SOUNDS LIKE YOUR BANGING ON A TIN CAN. WHEN THAT HAPPENS THE TRUCK IS NOT STOPPING OR SLOWING AT ALL AND THE BRAKE PEDAL IS AS FAR DOWN AS IT CAN GO. PUMPING THE BRAKE WILL CAUSE THE BRAKES TO RECOVER AND THE VEHICLE WILL STOP. THIS PROBLEM HAS NOT BEEN A CONSISTENT ISSUE. THIS LAST THURSDAY IS THE FIRST TIME I COULDN?T STOP IN 26000 MILES. BACK WHEN THE VEHICLE HAD 15000 MILES ON IT THE SAME THING OCCURRED. WHILE DRIVING THE VEHICLE TO WORK THIS MORNING AFTER PICKING IT UP FROM THE GMC DEALERSHIP THAT SAID THERE WAS NOTHING WRONG IT HAPPENED AGAIN. THIS WAS AFTER MAKING SEVERAL STOPS ON HWY 280 IN BIRMINGHAM, AL. THIS TIME I RAN THE RED LIGHT WITH NO CONSEQUENCES BUT MY LUCK WILL NOT CONTINUE. *TR

SEVERE STEERING WHEEL VIBRATION WHEN BRAKING AT 40 PLUS MPH. *TR

VIBRATION IN FRONT BRAKES, WHEN BRAKING AROUND 55 - 60 MPH. *TR