I did rear CPR on a 2003 Jetta 1.8T, they were metal on metal. Car came back metal to metal within several months. Replaced it all again but now have a hissing inside the car from the brake booster behind the pedal assembly and erratic brake operation. Replaced the booster with a used one and the brakes feel better but unable to get rid of the hissing. Replaced it again with a second used booster and the hissing will not go away. What am I doing wrong?