Rear Brake Caliper Piston Not Depressing

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by Seth the Younger, May 13, 2025 at 12:23 AM.

  1. Seth the Younger

    Seth the Younger Junior Member

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    I serviced my rear caliper pins today and changed the brake pads on the driver side caliper. I have the little magic cube to depress the caliper piston, but no matter how much I spun it, it would not depress at all. I tried to apply as much pressure as I could to help it engage, but no luck. After spinning it plenty, I thought maybe it could reverse threaded even though I was almost certain it wasn't. It started to come out even further immediately, but now it won't go back in even to the original spot.

    It seems like the screw must be engaged because it backed out just fine when I spun counter-clockwise. I thought maybe there was too much hydraulic pressure to overcome and the screw jack thing might slip a little by design. I made a half-hearted attempt to relieve some pressure in the caliper (I just cracked the bolt connected to the flexible hose and let it drip a bit), but I wasn't able to get it to depress with that either.

    Tomorrow, I will plan to rent one of the tools that compresses the piston as it spins it. I might also bleed some more brake fluid if that doesn't work. My next step would probably be to disconnect the caliper entirely and remove the piston (I imagine it comes out though I'm not sure).

    Anybody have any thoughts? Any similar experiences? I read this post ( 2015 Prius 2 rear caliper piston won’t retract | PriusChat ), but it seems like they were able to get it with the cube which is not the case for me.

    Thanks for all the help; with this and over the years of posts.
     
    #1 Seth the Younger, May 13, 2025 at 12:23 AM
  2. Tombukt2

    Tombukt2 Senior Member

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    I don't know I'm not sure isn't that a fiber piston in that caliber maybe the threads are stripped you're lucky it'll come loose but it won't go back I guess I don't know with pressure going it should run right down without any problem barely with a tool a makeshift to leave and I have the proper tool I'm just saying it can be done without it when it's working it doesn't sound like you have pressure problems or anything like that it sounds like you've got a thread catching problem something's not winding down like it should I mean there's nothing there I don't think I don't have a rear caliper apart
     
    #2 Tombukt2, May 13, 2025 at 11:22 AM
  3. Mendel Leisk

    Mendel Leisk Senior Member

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    This is 3rd gen, with manual parking brake? I used a “cube” on a ratchet, when pressed firmly and rotated clockwise it screwed the piston in without drama. Something sounds amiss in your case.

    there’s a spring in there, that acts like a kind of “Chinese Finger Trap”, will only rotate in one direction, cinches up if you rotate the other direction. I’ve never got my head around how things work in there, but maybe something’s failed internally.

    @ChapmanF may be along with more substantive ideas/explanations.
     
    #3 Mendel Leisk, May 13, 2025 at 12:28 PM
    Last edited: May 13, 2025 at 12:36 PM
  4. Tombukt2

    Tombukt2 Senior Member

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    Yeah I've never unscrewed one and taken the boot off and pulled it completely apart and not sure why rear brakes you know never really too interesting I guess. Now the front's thousands of times but the rear is not so much
     
    #4 Tombukt2, May 13, 2025 at 1:39 PM
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