1. Attachments are working again! Check out this thread for more details and to report any other bugs.

Hissing, puffing sound when pressing brake pedal

Discussion in 'Gen 3 Prius Technical Discussion' started by Steelholder, Oct 6, 2023.

  1. Steelholder

    Steelholder Junior Member

    Joined:
    Oct 25, 2022
    70
    17
    0
    Location:
    Encino
    Vehicle:
    2010 Prius
    Model:
    Three
    Hey all, I have a 2010. I'm getting a slight hissing puffing sound when the brake is pressed. Noise goes away when I press the pedal all the way down. It's obviously some kind of leak, is there a known or common issue here? It's really tight down there and would prefer to go straight to the problem if there is a common culprit, thanks!
     
  2. bisco

    bisco cookie crumbler

    Joined:
    May 11, 2005
    107,406
    48,778
    0
    Location:
    boston
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    the brake actuators are known to go bad, no trouble lights?
     
  3. Steelholder

    Steelholder Junior Member

    Joined:
    Oct 25, 2022
    70
    17
    0
    Location:
    Encino
    Vehicle:
    2010 Prius
    Model:
    Three
    None at all
     
  4. Steelholder

    Steelholder Junior Member

    Joined:
    Oct 25, 2022
    70
    17
    0
    Location:
    Encino
    Vehicle:
    2010 Prius
    Model:
    Three
    None at all
     
  5. bisco

    bisco cookie crumbler

    Joined:
    May 11, 2005
    107,406
    48,778
    0
    Location:
    boston
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    is the sound down at the pedal, or in front of you under the hood?
     
  6. ChapmanF

    ChapmanF Senior Member

    Joined:
    Mar 30, 2008
    22,782
    14,807
    0
    Location:
    Indiana, USA
    Vehicle:
    2010 Prius
    Model:
    IV
    Some get a hissing or puffing sound because of gas bubbles mixed in with the brake fluid. When you apply the brakes, more fluid than usual has to flow, to compress the bubbles, before the brakes really move, and you hear that squish sound. Sometimes you also hear it in reverse when releasing the pedal.

    The obvious reason there can be gas bubbles in there is if somebody worked on the brakes and had the system open and didn't completely bleed the air out afterward. There's also a less-obvious reason, because there is a pressure accumulator in the system that holds some pressurized nitrogen, and sometimes if that is worn it can slowly leak nitrogen into the fluid. Then you have mystery gas bubbles in there even though nobody had the system open.

    Going through the brake bleeding procedure (the full one, including the underhood stuff) is effective for getting the gas bubbles out, no matter how they got in. I recommend it. You don't really want bubbles in there.

    If they are there because of work someone did without bleeding properly after, they will probably be gone for good after proper bleeding.

    If they are there because of a slow nitrogen leak from the accumulator, you might get to do it again at some future point, but depending on how slow that leak is, it could be good for a while.