WHILE GOING INTO A PARKING SPACE, I ATTEMPTED TO ENGAGE THE BRAKES, THE BRAKE PEDAL DEPRESSED ENTIRELY TO THE FLOOR, AND THE BRAKES DID NOT ENGAGE. AS THE RESULT, MY VEHICLE DROPPED APPROXIMATELY 15 FEET OVER A RETAINING WALL TO A LOWER PARKING LOT LANDING NOSE FIRST ONTO THE PAVEMENT OF THE PARKING AREA BELOW. THE AIRBAG DEPLOYED AND THE VEHICLE ULTIMATELY LANDED ON ALL FOUR TIRES, ROLLING FORWARD UNTIL I WAS ABLE TO STOP THE VEHICLE BY ENGAGING THE EMERGENCY BRAKE.