What causes the hissing sound when stepping on the car brake?

[StEllie]

Car brakes making a hissing sound may be caused by the vacuum booster. Below are the reasons for the hissing sound when stepping on the car brake: Long-term wear: Some vehicles always make abnormal noises when stepping on the clutch pedal, which may be mistaken for a clutch malfunction. This phenomenon occurs due to slight wear on the clutch pedal support shaft from long-term use. Generally, applying some lubricant to the support shaft position can resolve the abnormal noise. Determining the type of fault: Start the engine, run it for 1-2 minutes, then turn it off. Step on the pedal three times. With a normally functioning vacuum booster, the first step will have normal pedal travel because the vacuum booster has sufficient vacuum. The second step will have significantly reduced pedal travel because some vacuum has been lost in the booster. By the third step, there is very little vacuum left in the booster, so the pedal travel is minimal, and it becomes difficult to press further. This situation indicates that the brake booster pump is leaking, causing the sound.