What Causes the Hissing Sound When Braking?
2 Answers
The reasons for hearing a hissing sound when braking are as follows: 1. When you press the brake pedal, you can clearly hear a leaking sound, and sometimes a hissing sound after pressing the pedal. In this case, the initial judgment is that there might be an issue with the brake booster. 2. The leaking sound generally falls into two scenarios: one occurs during the normal operation of the brake booster but is only brief; the other happens after pressing the pedal, with a more noticeable or continuous sound. Additional Information: 1. To check if the sound is caused by a leaking brake booster: Start the engine, let it run for 1~2 minutes, then turn it off, and press the pedal three times. For a normally functioning vacuum booster, the first press will have a normal pedal travel due to sufficient vacuum in the booster; the second press will show significantly reduced travel as some vacuum has been lost; by the third press, with very little vacuum left in the booster, the pedal travel will be minimal, and further pressing will be difficult. 2. This is the so-called "each press higher than the last." It proves the booster is not leaking and is functioning properly. If each pedal press has minimal and unchanged travel, known as "extremely hard pedal," it indicates the booster is leaking and failing. Severe leaks can produce audible sounds, and a leaking booster should be replaced.
I have the same issue when driving - hearing a hissing sound like the car is sighing whenever I brake. This usually indicates a problem with the brake booster system, specifically air leakage from the vacuum hose or booster, causing air to escape during normal operation. The vacuum system relies on engine assistance to make braking easier, and when it leaks, not only does the sound occur, but in severe cases, the brake pedal becomes stiff and hard to press, which is extremely dangerous, especially in rainy conditions or at high speeds. Last time my car had this issue, I thought it was minor and delayed fixing it, but during an emergency brake, I almost rear-ended another car - luckily nothing serious happened. I immediately took it to the repair shop, and they found the booster connection was loose - just needed a small part replacement. Now I pay attention to any unusual brake noises and make sure to check these areas during regular maintenance. Safety first - don't try to save money or cut corners.
