brake pedal

Discussion in 'Gen 2 Prius Main Forum' started by GeorgeMW, Jun 20, 2018.

  1. GeorgeMW

    GeorgeMW Junior Member

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    Hello, i own a '06 prius, with 87k miles, there is one thing that happens only when trying to start the car, when i press the brake pedal, it pulls itself up violently, what i want to know is why this happens and how i can fix it, thank you all in advance!
     
  2. ChapmanF

    ChapmanF Senior Member

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    I can't recall ever seeing a behavior quite like that in all my days on PriusChat, but it seems like there can't be very many different explanations.

    Looking in the diagram in your New Car Features manual, of all the solenoid valves in the actuator, there are two of them, SMC1 and SMC2, that control whether your master cylinder is really connected to the brake lines or not. In normal operation, it isn't: you are only pushing fluid into a "stroke simulator", and the computer controls the other valves (the SLA* and SLR* valves) to send the pressurized fluid to the brakes.

    In "fail safe" operation, or when the power is off, SMC1 and SMC2 are open (they are the kind that open when there's no power), so your master cylinder really is connected to the front brake lines and you directly apply the front brakes when you push the pedal.

    If you are getting in and starting the car, and for some reason SMC1 or SMC2 is sluggish about closing and isolating your master cylinder (maybe they're just old and sticky?), then when you press the brake pedal and the computer opens SLAFL and SLAFR to apply the 2000 psi accumulator fluid into the front brake lines, it would be able to flow up to the master cylinder and whomp the pedal back atcha.

    And there probably, really, isn't any other story that would explain what you describe.

    Anyway, while it's an interesting puzzle to work out what exactly's going on there, it isn't as if you can go and buy SMC1 and SMC2 individually ... the actuator is only sold as a complete unit.

    -Chap
     
  3. GeorgeMW

    GeorgeMW Junior Member

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    thanks a lot for your reply, Chap, but it seems like its best to have the car checked...
     
  4. GeorgeMW

    GeorgeMW Junior Member

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    Oh and i forgot to add the fact that after the pedal pulls itself up, it goes back down pretty quickly
     
  5. ChapmanF

    ChapmanF Senior Member

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    ... as would happen if SMC1/2 had been sluggish about closing, but eventually did. Your described effect would just be the result of them not managing to completely close before the fluid pressure is applied.

    As I said, it's an interesting case and fun to work out, but yes, to get your car fixed you will have to have it checked and repaired.

    -Chap
     
  6. fotomoto

    fotomoto Senior Member

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    +1 This is strange. Hope the OP lets us know what they find out.
     
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  7. Paul OByrne

    Paul OByrne Junior Member

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    What he is describing is one of the same symptoms that I also have- due to what I believe is a failing brake actuator. I have DTC's C1256 and C1391. These state that the Accumulator is leaking or low on pressure. The brakes act like they no longer have power and you need to push down hard on the pedal to brake. There are also additional pumping sounds- like the system is trying to keep the pressure up but is struggling to do so.
    I'm going to ask the dealer about the warranty enhancement but expect I may be out of luck on that.
    I have researched these threads on this and still looking for someone that has done this repair or has any insight on the procedures. Any videos, good tips, instructions are very welcome.
    Thanks,
    Paul
     
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