I WAS DRIVING DOWN THE ROAD WHEN I WAS COMING TO A STOP MY BRAKE WOULD NOT STOP ME. WHEN I PUSH ON THE BRAKES YOU WOULD HEAR A WHISTLE SOUND. CYLINDER AND BOOSTER BOTH REPLACED AND PEDAL/BRAKES ARE STILL SOFT. AFTER HIT A BUMP, THE BRAKES WOULD GO COMPLETELY SOFT AND REQUIRED THEM TO BE PUMPED UP. THIS HAPPENED SEVERAL TIMES. WE REPLACED THE MASTER CYLINDER AND BOOSTER AFTER BRAKE PEDAL WOULD GO ALL THE WAY DOWN WITH LITTLE STOPPING POWER. REPLACING THE MASTER CYLINDER FIXED THAT ISSUE, BUT NOW THE PROBLEM IS EXACTLY AS DESCRIBED IN THE PREVIOUS : BRAKES BEGIN TO ENGAGE, THEN GO SOFT (SPONGY) FOR A BIT, THEN RE-ENGAGE. BRAKING POWER IS SIGNIFICANTLY LESS THAN BEFORE.