WE WERE IN STOP AND GO TRAFFIC ON THE HIGHWAY FOR JUST OVER A HALF HOUR WITH MY TWO YOUNG CHILDREN IN THE CAR. SPEEDS BELOW 10 MILES PER HOUR. AT THE TIME OF INCIDENT, WE WERE TRAVELING BETWEEN 5-7 MPH. TRIED COMING TO A STOP WITH ABOUT 10 FEET BETWEEN OUR CAR AND A BIG RIG IN FRONT. THE BRAKES WOULDN'T EGANGE. REPEATEDLY STOMPED ON THE BRAKE PEDAL BETWEEN 5 TO 10 TIMES. CAR BREAK WOULD NOT ENGAGE AND WE ROLLED INTO BIG RIG BUMPER. THIS GAVE US A CHANCE TO POWER DOWN THE CAR WITH THE PUSH/START BUTTON.