already replaced master cylinder and rebuilt booster. and my brakes the pedal when the car is turned off, they work. (the pedal is hard and doesnt hit the floor). when car is on, the pedal goes all the way in and an air sound is heard... any ideas????

1.8 t

 

Sounds like either the booster or a vacuum leak. Can't imagine what else would be related to the engine. Have you verified that the brakes are actually working with the engine off, or just that the pedal is hard? What happens if you disconnect the vacuum line from the booster?

 

thanks for replying. yes the brakes work. and when i take out the hose, the car turns off

 

you need to plug the hose when you take it off! if not, like you said, the car will turn off.

try it again with the hose plugged somehow