that'd be you brake booster.
when the brakes are applied the booster equalizes pressure through a vent inside the cabin at the pedal pushrod. There's supposed to be a little foam 'muffler' in the vent. I think they deteriorate.
perfect little animation to show you
http://auto.howstuffworks.com/power-brake2.htm
when the brakes are applied the booster equalizes pressure through a vent inside the cabin at the pedal pushrod. There's supposed to be a little foam 'muffler' in the vent. I think they deteriorate.
perfect little animation to show you
http://auto.howstuffworks.com/power-brake2.htm