I recently noticed that when applying the brakes, I can hear a hissing sound that is similar to air being released. The car is a 2006 Infiniti g35x. I did a thorough search and the problem that comes up says brake booster issue. However, I did some tests with the pedal and turning the car on/off and seeing if the pedal goes down a little. There are no other issues that point to the brake booster being the issue. The only problem is this hissing sound. I have had the brake pads and rotors replaced within the last few thousand miles, so they aren’t worn. I am hoping that this sound is no big deal. There was no visible issue under the hood that I could find. My fluid was at a normal level, the sound only occurs when the car is moving. If it’s a few mph or stationary, there is no sound when pressing the brake. Any help is appreciated. Thanks

  • Welcome to Motor Vehicle Maintenance & Repair! Apr 25, 2019 at 1:20

1 Answer 1


The test you conducted checks to see if the brake booster is working, and not leaking with the pedal released.

The vacuum boost section has a leak and will not seal on the push rod when the pedal is depressed.

The leak is not sufficient to prevent booster operation, so there will be no other obvious indication.

You can disconnect the vacuum hose at the booster (and plug the end) to see if the noise goes away.

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  • So are you suggesting that the vacuum hose has a leak and that is the issue? I believe that there’s a large price difference in repairing the two... still a little confused.
    – alex 23
    Apr 26, 2019 at 21:15
  • 1
    No, I'm suggesting the large diaphragm inside the booster is leaking where the pushrod passes through it.
    – SteveRacer
    May 3, 2019 at 2:30

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