TRAVELING ABOUT 40 MILES PER HOUR ON I-17 SB IN PHOENIX. VEHICLE IN FRONT OF ME HIT BRAKES SUDDENLY. WHEN I SLAMMED ON MY BRAKES,IT DIDN'T FEEL LIKE MY BRAKES WERE WORKING PROPERLY. BRAKE PEDAL WENT ALL THE WAY TO THE FLOOR, DIDN'T SEEM TO BE STOPPING CAR. FELT LIKE THERE WAS AN ISSUE WITH THE BRAKES.