PROBLEM WITH MASTER CYLINDER POWER BOOSTER PUMP, VEHICLE FAILS TO STOP WHILE MOVING AT SHORT DISTANCE AND LOW SPEED 15 MILES OR LESS HIGHWY AND CITY. VEHICLE STOPS WHILE MOVING AT LONG DISTANCE AND HIGH SPEED 15 MILES OR MORE HIGHWAY AND CITY. BRAKE PEDALS VIBRATE AND GO TO BOTTOM AT LOW SPEEDS 15 MILES OR LESS HIGHWAY AND CITY.