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??????

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2 answers
booster is leaking and must be replaced.

Roy
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).