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Brake Accumulator Discussion

Discussion in 'Gen 2 Prius Technical Discussion' started by jromano89, Sep 4, 2015.

  1. jromano89

    jromano89 New Member

    Joined:
    Sep 4, 2015
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    Location:
    Pennsylvania
    Vehicle:
    2007 Prius
    Model:
    II
    Hello, I have a 2007 Prius with 165k miles that is frequently running the brake actuator pump (after roughly 3 stops)

    I did some testing with the Techstream software and found that the brake accumulator sensor normally sits at 3.5-3.6 volts and doesn't budge over time. However, each time the brake pedal is depressed, this drops by about 0.1 volts and doesn't return when the pedal is released. Once the voltage drops below 3.2 volts, the pump runs to bring it back up to 3.6 volts.

    I'm assuming the whole accumulator assembly will need to replaced eventually; but I'm wondering exactly what part is failing, and if there are any alternative fixes available. Am I correct in thinking that a solenoid must not be returning pressure to the accumulator properly from the calipers? Or is excess pressure being lost during braking?

    It's been hard to find clear information for this online, so any input is appreciated. Wondering what normal operation should look like. Thanks
     
    #1 jromano89, Sep 4, 2015
    Last edited: Sep 4, 2015
  2. ChapmanF

    ChapmanF Senior Member

    Joined:
    Mar 30, 2008
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    Location:
    Indiana, USA
    Vehicle:
    2010 Prius
    Model:
    IV


    Normal operation looks exactly as you've described. The accumulator pump takes fluid from the (unpressurized) reservoir and forces it into the accumulator to reach a certain pressure. With every use of the brakes, some of that fluid is used, and returns to the (unpressurized) reservoir when the brakes are released. When the pressure reaches a low threshold, the pump runs again.

    So that's completely normal as far as it goes. How long have you watched without braking, for your observation "3.5-3.6 volts and doesn't budge over time"? If you had a slow leak, it would either be external (you would notice brake fluid somewhere) or internal (just leaking slowly past a seal or the relief valve and returning to the reservoir). To a certain degree that's normal; mine usually needs to pump itself up first thing in the morning. It could also be something that increases over time.

    You have a US Gen 2, so rear drum brakes, right? How are the self-adjusters working there? If they get jammed, the clearance between shoes and drums increases slightly, increasing the amount of travel when you apply the brakes. That would slightly increase the amount of fluid used up per use of the brake, and returned to the reservoir. In that case, getting the rears adjusted might get the behavior back the way you remember it.

    If the power to the brake pump is off, how many pedal strokes do you get before the pedal gets hard? (This test doesn't require great sensitivity; it gets really hard when the accumulator pressure's used up. Edit: I should be careful; Gen 1 gets really hard when the accumulator pressure's used up, but Gen 2's master cylinder is built differently and might make this test less useful.) It's an easy test on Gen 1 where just turning the key off is enough to keep the pump from running. On Gen 2 I think you actually have to pull something.

    I usually get 20-some to 30-some strokes....

    -Chap
     
    #2 ChapmanF, Sep 4, 2015
    Last edited: Sep 4, 2015
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