Brake system pressurizing while driving?

Discussion in 'Gen 3 Prius Main Forum' started by zm15, Apr 19, 2012.

  1. zm15

    zm15 Junior Member

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    So i know that when you open the driver side door after the car has been sitting awhile (2010 Prius), it makes a whirring noise and that's the brake system pressurizing before you start the car.

    But this whirring continues as i drive the car at lower speeds within 5-10 minutes of starting on my way to work.

    The first mile is pretty much down hill, only need the ICE for about 1/5 of that distance, and use regen braking all the way. When i come to a stop sign - there are 3 in that first mile - the whirring sound kicks in. I notice it most when i'm stopped, then slowly creep forward as the car ahead of me goes through the stop sign and it's my turn to stop, i'll hit the brakes for that last bit and hear the whirring.

    Is this normal after the has been started and driven for a short bit?
     
  2. qbee42

    qbee42 My other car is a boat

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    It isn't driving that makes the whirring, it's braking. Each time you press the brake pedal it uses some pressure from the brake system. Eventually this pressure is restored by the brake accumulator pump, which is that whirring you describe.

    The pump running every third time or so is normal. It should not run continuously.

    Tom
     
  3. zm15

    zm15 Junior Member

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    Thanks Tom, it doesn't run every time i brake that i recall, and it doesn't seem to do it after i've driven for a bit.

    If it runs every time, what does that mean?
     
  4. qbee42

    qbee42 My other car is a boat

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    If it runs continuously it means your brake system has a pressure leak.

    Running every time you brake could be nothing more than your braking habits.

    Tom
     
  5. zm15

    zm15 Junior Member

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    ahh, perhaps i misunderstood what i read, oops. I read that as: 'continuously meant that it happened every time i hit the brakes', instead of meaning 'running for the duration while the car is on, not just while pressing the brakes'.


    and no, it only happens when i press the brakes upon coming to a stop, not all the time.

    that's clearer for me now that i've had my coffee! :)
     
  6. David Beale

    David Beale Senior Member

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    Your car isn't old enough yet, but it -can- run -every single time- you press the brakes. This indicates the pads are worn and the system has to move them farther, taking more volume (it's not pressure, it's volume that triggers the use of the system pressure - in other words, if your system only had to move 1 cc of fluid instead of 10 cc the pressure would drop much less and the pump would run less).

    Oh, and you're much too sensitive to the sounds the Prius makes. I recommend loud rock music with earbuds to remedy this. ;)
     
  7. zm15

    zm15 Junior Member

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    yeah pretty sure the pads aren't worn. I better understand what's going now.

    and yes loud music helps!!
     
  8. Constance83

    Constance83 New Member

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    [​IMG]Thanks Tom, it doesn't run every time i brake that i recall
     
  9. xs650

    xs650 Senior Member

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    Disc brakes self adjust with an ingeniously simple system. They don't move further when they are worn.
    This will save me a bunch of typing
    Disc brake calipers
     
  10. David Beale

    David Beale Senior Member

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    -When they work as they should-. Good luck with that.
     
  11. xs650

    xs650 Senior Member

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    In 46years of driving cars with disc brakes I have never had them not work. Square o-rings are pretty reliable. If they don't work, your pedal will go right to the floor when the pads wear a bit.

    Edit: A little clarification. I was referring to the self adjusting feature of hydraulic disc brakes, not the entire disc brake system.
     
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