Brake accumulator, and brake bleed. Has an Interesting result.

Discussion in 'Gen 2 Prius Technical Discussion' started by kens97uber171, Apr 14, 2018.

  1. kens97uber171

    kens97uber171 Junior Member

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    My brake accumulator pump has been running fairly consistently typical failure from what I understand.
    Started with the accumulator pump running continuously eventually started making some weird barking noises, followed by the ABS VSC warning lights have never received the red triangle however.

    If I do a thorough brake bleeding on the rear brakes the symptoms disappear for awhile, I still get accumulator cycling at a higher rate than normal usually. However the barking noises go away and the warning lights go away for awhile.

    What's interesting is when you do the brake bleed on the rears, when you crack the bleeder Port you get fluid coming out and it make some interesting noises while it's doing that, eventually the noise changes dramatically and you start to see air bubbles in the fluid stream... eventually they stop.
    If you repeat this cycle on both left and right
    The car behaves pretty normally for up to about a week
    So obviously air is getting into the system somehow, I don't have a leak any place, I don't lose any fluid, and I drive a thousand miles a week.. so I would think I would notice and fluid loss. I don't fully understand the fluid path from reservoir to master cylinder.. to accumulator out to the wheels. I
    know the fluid moves back and forth in some pattern so what I'm wondering is if it could be picking up air somewhere along that path??
    It's also possible I assume that this IS the failure modes at the accumulator pump demonstrates in which case it needs a new pump... which I have already purchased.
    I'm just wondering if there's not a loose hose or a loose pressure line or something or perhaps that air is getting in through the master cylinder I don't know would very much appreciate your input.
    I apologize for the long message here and also the video is about 20 minutes long, because I pieced
    several short videos together. The beginning of it is the brake bleeding procedure that's probably the most important thing if you don't want to watch the whole thing
    thank you very much for your help.
     
  2. ChapmanF

    ChapmanF Senior Member

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    I suspect you're not seeing any "air" coming out that hasn't been there since day 1. The accumulator works by pumping the fluid into a canister filled with highly pressurized nitrogen gas. You know how the accumulator assembly looks like it has two big round cylinders? One is the motor for the pump, and the other is the nitrogen canister.

    Inside there is a beefy, beautiful, unbreachable bellows to keep all the nitrogen on one side and all the brake fluid on the other side. I don't know whether Mexico paid for it. But anyway, after some number of miles and years and thousands of flexing cycles, well, you can start losing some of the nitrogen into the fluid circuits.

    Eventually, with less and less volume of nitrogen inside the can, the pump has to run more and more often for shorter and shorter cycles, trying to keep the pressure up. (If you have a well pump at home and your pressure tank is losing its air charge, the same thing happens.)

    -Chap
     
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  3. edthefox5

    edthefox5 Senior Member

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    Abs is leaking and Losing vac which is why the pump runs continuously.
     
  4. ChapmanF

    ChapmanF Senior Member

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    To recap #2 a bit, the Prius brakes do not use a vacuum system. They use a pressurized fluid system, with the fluid pumped into an accumulator precharged with nitrogen gas. The bellows inside there can fail eventually, allowing the high-pressure nitrogen gas into the hydraulic circuits.

    -Chap
     
  5. kens97uber171

    kens97uber171 Junior Member

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    Thanks for the description of how this works. I had no idea about the nitrogen filled part.

    Do you know of any good diagrams of the system.. also
    A good disassembly instructions or even better a video..

    I'm very mechanically inclined... I just like having as much visual information as I can before I start a complicated repair.

    Thanks again for all your help..
    It's very much allapprecia.

    Ken
     
  6. ChapmanF

    ChapmanF Senior Member

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    The diagrams of the system in the New Car Features manual on techinfo.toyota.com are exactly what you're looking for to understand the system.

    Once you have a techinfo login, all the other repair manual information is there too for your disassembly/replacement steps.

    -Chap
     
  7. kens97uber171

    kens97uber171 Junior Member

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    Thank you very much I will get those... And study up a bit before taking the car apart.

    Thanks again.
     
  8. jbct

    jbct New Member

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    Hello, my 2008 with 152,000 miles is doing this. What part(s) did you end up replacing?
     
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