When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Sounds like either the booster or a vacuum leak in the booster hose.
With the car parked and running, have a friend try the brakes while you listen in the booster area with the hood open. Most of the time you can hear the hissing where the leak is to pinpoint if it's the actual booster or the hose. You may get lucky and the hose is just loose.