His coming from under brake pedal, redux
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  1. #1
    Join Date
    Aug 2011

    Default His coming from under brake pedal, redux

    A couple of years ago I posted about an undiagnosed hissing noise that's coming from under the brake pedal of my 2002 Scenic Dynamique (auto.) It's been happening for six years or more. The fine people on this forum tried to find an answer, and no one had heard of this happening. The local Euro repairer couldn't sort it out, and didn't really want the work anyway. Two separate Renault dealers had no idea, either, and both insisted the brake booster needed to be replaced. The brakes are not affected in any way, the car still brakes straight and true, and the pedal doesn't travel to the floor.

    We did however come across someone who had a look and discovered that the rubber "diaphragm" (not sure if this is the correct term) under the brake pedal had simply come loose and needed to be reattached. He also showed us a youtube video of the same issue, complete with little whistling noise. Unfortuntely he wasn't able to reattach it because it also has a small split in it. Such a simple fix to an issue that even the dealers couldn't diagnose.

    Now comes the task of trying to find a new/second hand brake pedal, with roadworthy tread and intact diaphragm. Any and all ideas to locate parts gratefully accepted.


    Sorry, I just noticed the typo in the title. HISS
    Last edited by Cecile; 26th September 2014 at 02:46 PM.

  2. #2
    1000+ Posts jo proffi's Avatar
    Join Date
    May 2003


    Both my renaults have always, and probably will always make that noise.


  3. #3
    1000+ Posts schlitzaugen's Avatar
    Join Date
    Nov 2010
    loneliness capital of the world


    Is this the video?


    I now understand where the hiss comes from. What you have is basically a vacuum leak on the side of the booster diaphragm that helps the MC push the pistons out after you brake. That is why your brakes feel the same for you. They may not feel the same for your master cylinder trying to push the pistons back against atmospheric pressure. Not that it can't do it (given that pressures inside the MC are way above atmospheric), but who knows?

    The more important question is does the control valve in your booster stick open after you brake? Because if it does, then in the rest position there is a vacuum leak to the engine. This may not make the brake feel hard (yet) for you, but if the control valve sticks open further (to the point that there is a serious vacuum leak) it might. Your engine might show bad running too then.

    If the control valve does not stick open, then all that vents is the vacuum behind the bellows in which case the hissing should cease after a while (atmospheric pressure is restored under the bellows). If this is the case, you can simply remove the bellows altogether but you're running the risk of getting crap inside and get the booster/valves well and truly verschnickered.

    It would be interesting to check if there's hissing with the engine not running (as in the video above).
    Last edited by schlitzaugen; 30th September 2014 at 04:52 PM.

    Das computermachine is nicht fur gefingerpoken und mittengrabben. Ist easy schnappen der springenwerk, blowenfusen und poppencorken mit spitssparken. Ist nicht fur gewerken bei das dummkopfen. Das rubbernecken sightseeren keepen hands in das pockets-relaxen und watch das blinkenlights.

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