Why do my pistons need screwing?

Discussion in 'Car and Van Talk' started by Deleted member 33931, Sep 23, 2017.

  1. On my old Zafira and now on my 'new' Picasso, the rear brake calliper pistons MUST be screwed back in to the callips to accept fresh pads.

    Why is this?

    Some say... it's to prevent the O rings being damaged/folding over themselves if the pistons are just pushed back in (but why should rears be different from fronts?), and others that it's to do with the handbrake also being on the disc brakes; the handbrake works by turning the piston housing so that it 'screws' out when operated, and that it stays in these incremently-out positions as the pads wear - so it then all needs screwing back in.

    Any ideas?

    Thanks. Very curious.
  2. KIAB

    KIAB Super Member

    It's all to do with the handbrake, being actuated by the caliper . but I can't remember what else.

    Front calipers were always pushed back,some rear calipers were alway screwed back, other just pushed back.
    Deleted member 33931 likes this.
  3. Isitreally

    Isitreally Super Member

    You've answered your own question.
    It's to do with the handbrake, as the pads wear and the handbrake applied, it winds the piston out to take up the gap caused by the pads wearing.
    The foot brake applies hydraulic pressure to the rear of the piston to apply the brakes so can cover a larger gap if you like, but the handbrake is cable operated so if the gap was too big the cable movement would be excessive in taking up the gap and causing the handbrake to be ineffective.
    It's like the old and in fact still used drum brakes system in the sense of the shoes are adjusted automatically as they wear via a winding out adjuster on the handbrake system.
    Deleted member 33931 likes this.
  4. Thanks gents.

    Just done this job on my Picasso. I got an official workshop CD manual on t'Bay and this states to screw 'em in anti-clockwise.

    Got to the screwing bit - and they simply would not turn.

    Trawled the interweb for a definitive answer, and found lots of mentions of 'It must be screwed...', but not one saying with clarity which way...


    So I posted a plea on a Citroen site and went back out - time to get job done was running out. Tried them clockwise - and they started to move in smoothly. Drove them home, fitted new pads and jobbie jobbed.

    So the bludy Citroen manual was wrong.

    Answer to my forum plea confirmed it was clockwise - on both sides.

    Jeepers - a simple job made complex.

    Been trying to work out the details of how this works - although in basic terms it's quite obvious: the handbrake actuator on the back of the calliper passes its rotation movement to a threaded 'pin' that goes in to the cylinder, with an O ring sealing it. The piston either sits on this threaded pin, or else the piston sits over a sleeve which has this pin going in to it. Turning the pin moves the piston slightly forward to activate the parking brake.

    What I wasn't able to find was how this screwing-retraction works - how does the thread on the retraction tool seemingly match so perfectly the thread on the pin so that the piston moves in at seemingly exactly the right rate?! How does it take just the right number of turns? Why doesn't it bind?

    I can only conclude that the pin's thread is coarser than that of the tool, so that the pin gets more than enough turns given to it when the tool is used. The piston can then keep being rotated as much as it wants above this amount.

    Something like that.

    Anybody still reading...? :oops:

  5. btiw2

    btiw2 Screwfix Select

    I think I see what you're saying.

    Is this another way of looking at it: One rotation is the same for both the brake piston and the tool. If the tool performs one rotation then so must the piston.
    The question is then (in my mind) how far does the tool move (in/out) vs the piston. I'd never thought about that.

    The main use of this tool is to push a brake piston back in (when changing brake pads) - unless there's some other reason to use these things that I don't know about. Front pistons don't have the grooves so we can ignore those.
    So you'd want the pins on the tool to slightly disengage from the grooves when using the tool, as the alternative would be binding - so the thread on the tool must be at a shallower angle than the thread in the brake.

    So yeah, makes sense. You've given this a lot more thought than I ever had.
  6. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    Surely the retraction tool has sliding pins!
  7. WillyEckerslike

    WillyEckerslike Screwfix Select

    Complete guess work here but there is a lever operated screw within the piston which when turned (applying the parking brake) creates an outwards movement forcing the pad/caliper onto the disc and this is reversed when the lever is released pulling the cylinder back.
    At the same time the application of the normal brakes is predominantly outwards with the retraction provided by flexing of the seal and the piston sliding within the piston to take up pad wear. The thread of the parking brake mechanism has to allow the cylinder to move along it to compensate for the pad wear but it is still free to rotate one way, then the other, to work. When you replace the pads you are merely rewinding the piston along the thread. Once installed the brake pedal is pumped to take up the slack with the hydraulic pressure combined with the design of the thread being sufficient to move it - a bit like a one way valve.
  8. That's pretty much it, Willy.

    The 'issue' - as btiw2 has analysed - is how the threaded rod inside the calliper - wot does the pushing out via the parking brake - is rewound the 'right' amount by the set thread pitch of the rewind tool.

    I presume, being a coarser thread, it will always get more than enough turns from the rewind tool to re-home it fully, but the piston is also free to spin as many extra times as it's made to by the tool, to no ill effect. The threaded rod doesn't engage directly with the piston, but with a supporting insert behind it which is 'engaged' with the piston (turns with it) but can move in and out independently. So I suspect the 'insert' was sent all the way back on the threaded rod well before the piston was fully pushed back in to the cylinder.

    I have now confused myself, and I do not believe anyone is actually reading this...

    It was just one of these things that makes you wonder "I know I have to do this, but what exactly is actually happening inside the bludy thing....?"
  9. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    It'll be as you think. The thread on the threaded rod is an extended thread otherwise the handbrake lever would need to pull the lever at the hub multiple turns. And as the tool turns, the piston moves away faster than the tool moves forward - and sliding pins!
  10. Sounds like it, Mr Ha, but I don't get the 'sliding pins' bit.
  11. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    Where, and how, does your 'tool' fit?(and keep it clean)
  12. WillyEckerslike

    WillyEckerslike Screwfix Select

    Surely all the parking brake has to do is overcome the retraction of the piston by the flex of the hydraulic seal - and hold it there until released. Applying the brakes through the foot pedal causes the piston to rotate around the thread independently of the parking brake. At most the parking brake is going to cause the thread to rotate by a quarter - possibly much lower - along a coarse acme type thread. I think that DA is referring to the 'tool' within the parking brake mechanism and I assume you're referring to the tool used to wind the piston back in during a pad replacement - sliding pins allowing for fitting to different size or makes of brakes.
  13. Ah! I see what you mean.

    Yes, the tool has a couple of pins that engage with the piston front, and that's what forces it to rotate when being rewound.

    I was trying to figure out what's going on inside the calliper during normal operation - I think Will has cracked it on his last post.
  14. I think you've cracked it, Willy :)

    I was aware of how the pistons themselves work - the rubber O ring has enough springiness in it to pull the piston back that tiny fraction of a mm each time the foot brake is released but, when the pad wears enough, the piston then also slides that extra tiny fraction further out and overcomes that springiness and stays popped out a teeny bit more to its new position. Where it resumes its springiness. That's how the piston part works.

    But what you add about how that threaded rod works makes total sense; I was fixated on it being the parking brake action that made the piston move out in incremental steps along that threaded rod. But your suggestion that it's simply the action of the foot brake - the hydraulics, spinning the piston (a teeny tiny microscopic bit each time) further out along the threaded rod that nails it.

    That soooo makes sense. Brilliant - thank you. :D


    I can now focus on something else.

    From Brakes to Bre...
  15. WillyEckerslike

    WillyEckerslike Screwfix Select

    Can't you give that one a brake?
    Deleted member 33931 likes this.
  16. Isitreally

    Isitreally Super Member

    In nearly, if not all cases, the handbrake adjusts the pistons.
    It a simple case of the handbrake is the section of the system that needs the pistons to adjust so it makes sense that it adjusts them.
  17. And that is the question here - the one that answers all the answers.

    What adjusts the pistons in this setup - is it the hydraulics or the parking brake?

    I suspect the former as Willy says.
  18. WillyEckerslike

    WillyEckerslike Screwfix Select

    If that were the case, wouldn't you have to repeatedly operate the parking brake to relocate new pads once they had been installed rather than pump the pedal?
    Deleted member 33931 likes this.
  19. Now that is a good point - it was def the foot brake that needed pumping to reposition the new pads.

    So there it is - the piston spirals up the threaded rod as the pads wear, like an old spinning top. All the hand-brake does is to give that rod an extra quarter-turn when needed for parking.

    Case closed :)

    Unless anyone knows differently...

  20. Agree.

    If you replace a caliper it is most important that the handbrake is not used until the footbrake has been pumped to set the pads in the right position.
    But what I still dont fully understand is that if the footbrake sets the position of the piston to the pad/disc and the handbrake just pulls the pads close manually, then why does it need to be threaded/screwed.

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