FAILURE IN BRAKE VACUUM PUMP CAUSED MY BRAKES TO IMPROPERLY WORK THIS ME REAR ENDING ANOTHER CAR. BRAKE FELT LIKE A ROCK AND PUSHED BACK AGAINST MY FOOT. I WAS IN MOTION AT ROUGHLY 15MPH AND WAS ABLE TO SLOW TO ROUGHLY 8MPH AT A DISTANCE OF ROUGHLY 10 YARDS. THIS WAS DURING RUSH HOUR TRAFFIC. PROBLEM HAD BEEN INTERMITTENT AT VERY LOW PARKING LOT SPEEDS.