Leaking Brake Fluid

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by andreimontreal, Apr 6, 2026 at 12:19 AM.

  1. andreimontreal

    andreimontreal Active Member

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    Early this winter, traction control, abs, parking brake ( red exclamation with circle and parantheses), and brake warning light ( yellow exclamation with circle and parantheses) turned on. Those came and went then they stayed on for good. I knew this rear wheel sensor was acting up, and I had the same lights, and I thought it was that again because I got the same lights last year.

    One time, the parking brake sign/light turned on for a min or so when I put the car in ready, then the other lights came on. That's when I realized I was losing brake fluid. My cup under low.

    I topped it; it takes 1-2 months on average for it to empty (hard to tell, I guess it depends on usage); I could swear I heard recently drops fall inside the bay on metal as I was stationary and depressing the brake. Recently, I heard the booster motor whirl for like 30-60 seconds or so. I topped it then it started doing its normal thing, short bursts. When I top it the lights go away btw and everything works fine.

    This makes me think the leak is at the brake booster or bellow it since it was so empty that the engine was running without compressing anything.

    I can't confirm 100%, but at least when the lights are on, the ABS (and of course the cruise control) are not working. Not sure if the ABS is working when topped off - but off the top of my memory I think it did the last time snow fell; I wasn't trying to actively slide. Early in the winter I did a few rounds in the parking lot and ABS was engaging.

    Of course I will address this asap. I am aware that if that tiny gap suddenly bursts I'm SOL right away. Pulling codes tomorrow. Some early questions:

    1) Are there any known regular failures that I should start with/ look for?

    2) Is there a possiblity that one of the pipes burst and that the booster/abs control are fine? Given that the brakes work normally once I top it off. Or when this happens the brake booster needs to be replaced.

    I'm not 100% sure yet how to troubleshoot this to pin point the issue. Once I open it I guess it will be more clear.
     
    #1 andreimontreal, Apr 6, 2026 at 12:19 AM
  2. rjparker

    rjparker Tu Humilde Sirviente

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    If you are losing fluid you have a leak. It is leaking. There are no paths into the engine like a vacuum operated system might have.. Find the leak.
     
    #2 rjparker, Apr 6, 2026 at 12:40 AM
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  3. Andy Pants

    Andy Pants Member

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    There is a well know issue with our brake booster/accumulator pump on 3rd gens, however when this is the issue you typically don't lose fluid as it is leaking into itself and can't maintain pressure basically. I'm thinking you need to trace all the lines and find the leak.
     
    #3 Andy Pants, Apr 6, 2026 at 12:42 AM
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  4. ChapmanF

    ChapmanF Senior Member

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    I had an old Ford once where a brake line to the rear had rusted and developed a split. It would only ever leak when braking so hard that the fluid pressure pushed the split open and fluid sprayed out. The rest of the time, no leak.

    There was no sign of leaking near that brake tube at all. There was just a spot wet with brake fluid clear on the other side of the underbody where the spray would hit when it sprayed out of the split line.

    I could confirm it by wrapping shop towels around the line and then stomping the brakes hard. The split was where the shop towel got wet.

    (Don't just go under the car and watch for something like that while somebody stomps the brakes; a jet of high-pressure fluid like that can get through skin and be a surgical emergency.)
     
    #4 ChapmanF, Apr 6, 2026 at 11:07 AM
    Last edited: Apr 6, 2026 at 1:34 PM
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  5. andreimontreal

    andreimontreal Active Member

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    Yeah, that's what I'm afraid of. I'm in for some real fun apparently. I got your warning loud and clear thanks.
     
    #5 andreimontreal, Apr 6, 2026 at 12:43 PM
  6. ChapmanF

    ChapmanF Senior Member

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    Well, don't make it out to be more daunting than it is; there really aren't all that many brake lines or connections to look at, and a place wet with brake fluid usually isn't too hard to spot.
     
    #6 ChapmanF, Apr 6, 2026 at 1:39 PM
  7. andreimontreal

    andreimontreal Active Member

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    #7 andreimontreal, Apr 6, 2026 at 3:43 PM
  8. ChapmanF

    ChapmanF Senior Member

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    C1202. C1256 (reservoir level, accumulator low pressure)
    C1238, C1466 (about right rear speed sensor signal)
    C1239, C1467 (about left rear speed sensor signal)

    P1578

    P1578

    I transcribed the trouble codes from your attachments for you—I recommend doing that, for a couple of reasons: (1) if you do it yourself (once), you save everyone reading your post having to make extra clicks just to see what codes you're talking about, and (2) it allows your thread to come up when people search by those codes.

    The speed sensor codes could reflect bearing wear, or something else could be going on like wire damage or bad connections. The troubleshooting steps in the manual might help narrow down the possibilities.
     
    #8 ChapmanF, Apr 6, 2026 at 4:25 PM
  9. andreimontreal

    andreimontreal Active Member

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    Agreed, my bad.
     
    #9 andreimontreal, Apr 6, 2026 at 6:21 PM
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