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Q: I hear a hissing sound when driving, and it only stops right after applying the brake pedal.

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I would just like to know the problem, and if I have to replace any parts. Thank you.
My car has an automatic transmission.

A likely cause of the hissing sound is a leak in the atmospheric valve within the power brake booster. The reason the sound ceases when you apply the brakes is because the vacuum valve within the booster closes upon pedal application, thus cutting off the air flow into the booster through the atmospheric valve where the hissing noise is being generated. If indeed this is diagnosed as the cause, in an examination by YourMechanic, the recommended fix would be to replace the brake booster. However, other sources of vacuum leaks would have to be ruled out as well. If you desire this to be inspected by YourMechanic, please fee free to follow up and request a hissing noise at brake pedal diagnostic.

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