What would cause my brakes to hiss when applied? on 2010 Ford Fusion

It started early today when I was half way to work. The brakes still work the same and the sound seem to have stopped??????

Asked by for the 2010 Ford Fusion
booster is leaking and must be replaced.

1 more answer
Wear sensors on brake pads touching rotor , makes noise like 'fingernails on a chalkbord' . If that more closely describes the noise , then have your brakes inspected , if the wear sensor broke off (pad(s) almost metal-to-metal) , might damage rotor(s).