Gen 4 ABS Master Cylinder Replacement

Discussion in 'Gen 4 Prius Technical Discussion' started by Iron Man Prius, Jul 31, 2025 at 9:03 PM.

  1. Iron Man Prius

    Iron Man Prius Junior Member

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    Update on Brake ABS master cylinder replacement on 2016 Prius. Installed nearly new used master cylinder due to actuator pump running too much in "ready" mode sitting still, and while braking. (Thinking it's similar to Gen 3 in how it works). I did not replace the accumulator/pump yet, just master cylinder.

    Afterwards, now in Ready mode in PARK..sitting still no braking there are no actuations from the pump after a minimum of 10 mins which I think seems good, but stepping on brake it activated every 2 presses and while moving more than average pump cycles a lot still compared to other Gen 4's of the same vintage I've tested. With car turned OFF the pump actuates almost EVERY press of the pedal. Again..seems too much! I can't believe this is so hard to diagnose here, the cars have been around for 9 years now.

    Could the system is still leaking pressure if it actuates that much and how much is too many?Other Gen 4 cars pumps are nearly silent and not re-pressurizing as much. These apparently are not the same as Gen 3 as the car gets 62 mpg easily, and there's no codes for the ABS...safe to drive it and forget about it? Just seems excessive.
     
    #1 Iron Man Prius, Jul 31, 2025 at 9:03 PM
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  2. ChapmanF

    ChapmanF Senior Member

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    How confident are you that you got all the air out in bleeding?

    Air bubbles cause more brake fluid to move every time you use the brake. Unlike the fluid, the bubbles are compressible, so the target pressure isn't reached until more fluid has been sent out from the accumulator. When you release the pedal, that much extra fluid gets returned to the reservoir as the bubbles expand again.

    Because of that effect, an incompletely-bled system uses more fluid from the accumulator with every use of the brake, and it takes fewer uses of the brake to use up enough accumulator pressure to run the pump again.
     
    #2 ChapmanF, Jul 31, 2025 at 9:57 PM
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  3. Iron Man Prius

    Iron Man Prius Junior Member

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    We used Autel with Tech Stream to do all the tests and bleeding after install, not sure how that would fail to do the job?
     
    #3 Iron Man Prius, Jul 31, 2025 at 10:06 PM
  4. ChapmanF

    ChapmanF Senior Member

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    Autel and Techstream? That's dedication. :)

    I'm not saying it failed to do the job, but of course there's human judgment at every step (did I get all the air out at this bleeder? am I ready to go to the next one?). So it's definitely possible to bleed brakes incompletely.
     
    #4 ChapmanF, Jul 31, 2025 at 10:09 PM
  5. Iron Man Prius

    Iron Man Prius Junior Member

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    slash "/" typo
     
    #5 Iron Man Prius, Jul 31, 2025 at 10:14 PM
  6. ChapmanF

    ChapmanF Senior Member

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    Which did you use?

    Autel makes some standalone devices, and also some Bluetooth devices that work with an Autel app on your phone.

    Techstream is Toyota's official software, runs on a Windows laptop.

    As far as I know, they both can do brake bleeding ok. I have used them both (at different times:)) to do the job.
     
    #6 ChapmanF, Aug 1, 2025 at 9:15 AM
  7. Iron Man Prius

    Iron Man Prius Junior Member

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    We have both devices but Autel was used on this. It was done as thouroughly as possible.
     
    #7 Iron Man Prius, Aug 1, 2025 at 12:24 PM
  8. ChapmanF

    ChapmanF Senior Member

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    I'm not sure there is such a thing as "as thoroughly as possible" for brake bleeding. It's like body work (which I also dislike). You could always have spent more time, run more fluid through, prepped and sanded and polished longer, ....
     
    #8 ChapmanF, Aug 1, 2025 at 2:02 PM
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