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    #1

    tight caliper pistons

    I've replaced the O-rings on front and rear calipers, cleaned up the pistons and cylinders. How tight should the pistons be? I can move them by hand about 1/2 to 3/4" with some difficulity. Is this normal? With the O-ring removed the pistons move freely, I cleaned the O-ring grove. With the O-ring installed, there does seem to be a vacuum or good seal when I plug the holes and push the cylinder in.
    Do I need to hone the cylinders? They don't appeared scratched.

    Appreciate any advice on this.

    Thanks

    Mister T
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    #2
    Hi. I don't know what kind of bike you have, but your discription of how the pistons move sounds about right.
    What really caught my eye was you mentioning if you should hone the cylinders. No! I remember looking in my factory manual ('79 GS1000E)and the cylinder inside diameter service limit was only 1/1000th" more than when new. The pistons also can only wear 1/1000th". So don't scratch the pistons or hone the cylinders, there simply is not enough "excess" material.
    I don't know what your service limits are your bike, but at least you have an idea now.
    And on the seventh day,after resting from all that he had done,God went for a ride on his GS!
    Upon seeing that it was good, he went out again on his ZX14! But just a little bit faster!

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      #3
      Keith, thanks for the reply, the bike is an 83 GS750ED. I think they might still be in spec. I used a Dremel polishing wheel (cloth) with a touch of jewlers rouge, on the piston and cylinder. Cleaned the piston & cylinder with brake cleaning fluid, then washed the piston and cylinder in new brake fluid. Reinstalled the O-ring, piston. These are still on the bench, checked them tonight couldn't move the pistons at all by hand. That could be bad news.
      I'll let you know how it goes when I reconnect and bleed the system.

      Mister T

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        #4
        Caliper and piston polishing

        Don't polish these parts. Like others said there isn't much material there in the first place. Just clean with brake fluid and reassemble. They will feel tight to your hand but you have to remember the pressure that your hand can make and that is multiplied many times over by the master cylinder. It will move your pistons easily and so long as the calipers have a little play in them when pressure is released, they are fine. Bob

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          #5
          Hopefully I haven't ruined the calipers. Even though the pistons are difficult to move by hand, does the master cyclinder pull the piston away from the rotor when pressure is released?

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            #6
            As much as I can explain, when the lever is released, the master cylinder piston returns fluid to the reservoir. This allows the pads to back off the disc just enough.
            And on the seventh day,after resting from all that he had done,God went for a ride on his GS!
            Upon seeing that it was good, he went out again on his ZX14! But just a little bit faster!

            Comment

              #7
              Howdy,Ive been lurking on this site for a few days now and this post
              prompted me to go ahead and register and join the Group.
              Brake pistons with new seal rings should be very tight,if they go in
              easy you have a problem.
              Do not hone or sand pistons or caliper bores!
              Green Scotchbrite is sufficient to clean the caliper bore and piston.
              Minor pitting in the caliper bore is okay if it doesnt affect the seal groove.
              Pitting on the piston means new piston time.
              What initially retracts the piston is the resiliancy/stiction of the seal.
              The rest of retraction is due to the runout of the disc and the natural
              flex of the wheel in the forks/swingarm.
              Sounds to me like you should be in good shape.
              Hope this helps.
              Tim

              Comment

                #8
                Hi and welcome to the GS Resources site ! I appreciate the info on the calipers, I've never done this type of repair / maintenace before, I'll take all the help I can get. I did see pitting on the pistons, so it looks like new piston time, the bores looked ok. A fellow at work mentioned that I should hone the bores, I'm glad I asked here before I did that.

                Thanks again

                Comment

                  #9
                  Mister T,let me clarify the piston pitting problem.
                  If the pits are very small and LESS than the width of the "O" ring you
                  might not need to replace it.In use the piston moves across the o ring,
                  if you have pits at the last unused inside portion of the piston(caliper side)
                  and install new (thick) pads its OK.
                  Look at the portion of the piston thats been moving across the seal,
                  if its OK I would do it.
                  A pit as wide or wider than the seal will allow pressure/fluid to leak
                  across the seal inviting certain disaster!
                  Remember you only have two wheels to stop that bike,not four like
                  your mechanic buddy is used to working on.
                  Tim

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