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Q: Hissing noise when brake pedal depressed.

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Hissing sound when brake pedal is depressed. Fluid reservoir is full and there is no wetness inside under or near the pedal or carpet (I've even pulled the carpet up to look) so it doesn't look like I'm leaking fluid into the booster. The pedal is not hard to depress, and there's no smell or anything else. Just the hissing sound.

My car has 200000 miles.
My car has an automatic transmission.

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