What is the problem when there is a hissing sound during braking?
2 Answers
When there is a hissing sound during braking, it is preliminarily judged to be a problem with the brake booster pump. The hissing sound generally falls into two situations: 1. It occurs during the normal operation of the brake booster pump, but only briefly; 2. It is emitted after pressing the brake pedal, and the sound is more obvious or continuous. The following test can be conducted to determine the type of fault: 1. Start the engine, run it for 1~2 minutes, then turn it off, and press the brake pedal three times. 2. When the vacuum booster is working normally, during the first press of the pedal, due to sufficient vacuum in the vacuum booster, the pedal travel is normal; 3. During the second press, as some vacuum has been lost in the booster, the pedal travel will be significantly reduced; 4. By the third press, there is very little vacuum left in the vacuum booster, so the pedal travel is also minimal, and it becomes difficult to press further. This situation indicates that the hissing sound is caused by air leakage in the brake booster pump.
Last time I got my car repaired, the mechanic mentioned this issue. A hissing sound like air leaking from a tire is most likely a vacuum booster leak. This component is connected behind the brake pedal and creates vacuum to assist braking when you press the pedal. If its diaphragm is cracked or the hose is loose, it will leak air. For diesel vehicles, the issue might be with the vacuum pump, while gasoline cars often have a faulty one-way valve. Key areas to inspect: Check the vacuum hose for oil stains or cracks, and squeeze it to see if it feels soft; start the engine, pump the brakes a few times, then turn it off—if the pedal immediately becomes stiff, there’s a leak. Don’t delay fixing this—severe leaks can cause sudden brake stiffness, which is extremely dangerous.
