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Not My Day Today (hissing brake pedal now) :-(

Discussion in 'A3/S3 Forum (8L Chassis)' started by A3 T, Mar 5, 2011.

  1. A3 T
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    A3 T Lord Of The Rings

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    [Mar 5, 2011]
    It seems like I'm making a lot of threads today :-(


    Tonight i was out in the car with the radio off and i can hear a hissing sound coming from the brake pedal.

    When i got home i did a bit of googleing (as you do) and it seems that its a common fault with the brake booster (servo) only my problem has a bit of a twist.

    1. Its only hissing when my foot is off the pedal and as soon s i put my foot on the pedal it stops

    2. Vag-com isn't showing any faults with it

    3. No check lights on the dash

    4. No lumpy idle

    5. Brakes are really sharp


    All of this is the opposite to what I've been reading from people with same hissing sound coming from the brake pedal that ended up being the brake booster.


    Any ideas guys?
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  3. beachbuggy
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    beachbuggy Well-Known Member

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    [Mar 5, 2011]
    Well the brake booster works off the vacuum from the inlet manifold. If you press the brakes with the engine off the brake pedal feels hard, start the engine and it works as vacuum is used to assist the braking. Now if the the brake master cylinder has a leak then you will get air being sucked in, a hissing noise and possibly bad running. There is on the vacuum line a one way valve that stops boost going into the brake master cylinder, if this leaks then it can also cause problems. It would almost certainly sound as if the master cylinder or vacumm pipe is leaking and may be causing your other problems possibly.
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  4. A3 T
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    A3 T Lord Of The Rings

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    [Mar 5, 2011]
    Thanks mate ;)


    I'm going to try and investigate it a bit today

    The hissing sounds like the brake pedal is filled with air and has a hole in it and I'm stopping the leak with my foot....lol

    What other problems are you talking about :confused:

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  5. beachbuggy
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    beachbuggy Well-Known Member

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    [Mar 5, 2011]
    sorry I thought I saw another thread from you about rough idle, but it was my mistake it was another user. Please ignore that bit.
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  6. A3 T
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    A3 T Lord Of The Rings

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    [Mar 5, 2011]
    oh ok mate ;)


    I had a quick look today and I'm starting to think it could be normal if the servo is that close to the pedal.

    The hissing is not loud like an air compressor, its sound a bit like the radios not tuned in and you've got the volume really quiet.

    Its a little bit louder when your at idle and goes when you accelerate or apply the brake.


    My mechanic come back off holiday in a few days so ill be letting him have a look.


    But in the mean time keep the ideas coming and thanks for all the help ;)
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  7. beachbuggy
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    beachbuggy Well-Known Member

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    [Mar 5, 2011]
    Easy way to test.

    1) On the right hand side of the inlet manifold there is a pipe that runs to the brake servo. Remove this and either block it off or get someone to hold their finger over the port.
    2) Start the car, if the hissing has gone then it's the servo. If it's still there then it's something else.
    3) Make sure you re-attach the pipe!
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  8. A3 T
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    A3 T Lord Of The Rings

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    [Mar 6, 2011]
    Ok thanks mate, Ill try and give that a go tomorrow.


    Do you mean this hose?

    And should i plug both ends?


    [​IMG]
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