Why do my pistons need screwing?

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

  1. Isitreally

    Isitreally Super Member


    Correct, you do pump the brake to reset the pads, but you will find that you also need to pull the handbrake up and down a few times to reset it.
    The handbrake cable arm is the end of the rod that has the thread on i.
    It works on a ratchet system, not to dissimilar to the older style adjustment on Ford clutch cables from the 90s.
    As you pull the handbrake up it moves the arming in turn moving the rod pushing out the pistons, if there's to much free play it will ratchet the rod further out so the handbrake cable has the same amount of movement otherwise you be pulling it through the roof. ☺☺
     
  2. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    If it was a smooth pin, and the brake pad wears by 5mm, the pin would be 5mm away from the piston, needing a manual handbrake cable adjustment. Also, some other mechanism for the cable to work would be needed, as turning a smooth pin would do nothing. Not only that, there is a return spring on the lever that also releases the piston from touching the pad plate until the foot brake is again applied.
     
  3. Mr. Handyandy

    Mr. Handyandy Screwfix Select


    Not sure about that! Could end up with the handbrake almost applied without pulling it up.
     
  4. Isitreally

    Isitreally Super Member


    No you couldn't, cars have been doing this system for years.

    It's this way or brake shoes on the inside of the disc, like this.

    brake-drum-within-disc.png

    Or as VW used to do and have a lever system that force the calipar over engaging the pads on the disc.
     
  5. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    I understand it on the completely different brake shoes and hub.
    But surely not needed on the self adjusting screw system for pads!
     
  6. Isitreally

    Isitreally Super Member

    The self adjusting screw isn't for the pads, it's for the hand brake, the pads are held in the calipar and pushed one on side by the piston the other side is pulled by the slider.
    The piston is held at a set point by the brake fluid, the handbrake cable is a fixed length so when the pads wear you need something to extend that length so you have a threaded pin, this pin is on a ratchet system that alters the pin as the handbrake is applied as and when its needs to.

    That's How it works, no if or buts.
     
    Deleted member 33931 likes this.
  7. Mr. Handyandy

    Mr. Handyandy Screwfix Select


    But the pin doesn't need a ratchet system. The piston 'unscrews' nearer the pads(by means of the foot brake pedal) as they get thinner, so it is always near the pad!
    Unless of course you are saying that the handbrake cable NEVER needs adjusting?
     
  8. WillyEckerslike

    WillyEckerslike Screwfix Select

    If you watch this you'll see that there's no ratchet.
    There is however a screw which achieves what you're getting at to maintain consistent operation of the parking brake. If there was a ratchet, it would have to be released when new pads were required (as you do when replacing the shoes on a number of drum type operations similar to your earlier diagram)
     
    Deleted member 33931 likes this.
  9. Isitreally

    Isitreally Super Member


    No it doesn't, the piston is held in place by fluid,mthe pin is for the handbrake adjustment, as,I've explained the handbrake cable is a fixed length so you need a way of adjusting it to suit any wear on the pads, this adjustment is via the pin on a ratchet system.

    I'm not making this up I've done more than my fair share of brake on systems like this since the late 70s
     
  10. Isitreally

    Isitreally Super Member


    That on a VW calipar I've already explained above how they work, a totally different system.
     
  11. Isitreally

    Isitreally Super Member

    Also on that vid he even says the handbrake adjusts on the pin.

    The piston doesn't turn in its operation so what turns the pin?? yes the handbrake movement.
     
    Deleted member 33931 likes this.
  12. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    It must if you have to screw it back in for new pads!
     
  13. WillyEckerslike

    WillyEckerslike Screwfix Select

    I can see exactly what you're getting at however going back to your original notion that the parking brake does the adjusting, you would use that to reset new pads, not the hydraulics. Therefore it must be the hydraulics that take up the wear in the pads and the screw which adjusts when the parking brake is used to keep its operation consistent. I still haven't found a ratchet in any pistons. I have four cars of varying age and I do a lot of work on them and none of them has one. I think we're going to have to agree to differ for the time being. I'm working away for a week from tomorrow but will pop round and ask my mates at the garage when I get back as I'm really intrigued by this now.
     
    Deleted member 33931 likes this.
  14. WillyEckerslike

    WillyEckerslike Screwfix Select

    That was my first thought but I'm not sure. I'll check.
     
  15. Isitreally

    Isitreally Super Member


    Seriously. So it's wear a hole in the back of the pad while it does. :p:p
     
  16. Isitreally

    Isitreally Super Member


    The only adjustment needed is to the handbrake the hydraulics are just that, there's no adjustment to the foot brake on a system where the handbrake is independent.

    It's the same on drum brakes, the wheel cylinder pushes the shoes out to the drum, the handbrake adjusts them so they stay close to the drum, so when the handbrake is applied there's not lots of free play.
     
  17. Isitreally

    Isitreally Super Member


    Also when you wind the piston back you push fluid back into the system, you pump the pedal to move this fluid back taking up any free space.
     
  18. WillyEckerslike

    WillyEckerslike Screwfix Select

    That's not what you said earlier - #16
     
  19. WillyEckerslike

    WillyEckerslike Screwfix Select

    That would be taken up by your hand brake adjuster. As you adjusted the brakes using the parking brake the fluid would be sucked back in.
     
  20. Isitreally

    Isitreally Super Member


    It sort of is, the handbrake adjusts, not the hydraulics.
     

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