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    #31
    If the aftermarket master cylinder has a larger bore than the stock one, it can cause the symptoms that you are describing.
    1982 GS1100E "Jolene"

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      #32
      I matched the bore unless I measured incorrectly. In the picture from post #23 - I measured the diameter of the area in the top left of the pictures where you can see the top of the plunger (kind of underneath the mirror mount) - 15.8mm. Can somebody check to tell me if that is the correct way to measure?

      I couldn't verify with the plunger p/n: 59600-33811 (the MC plunger for my bike) since that one didn't fit in the MC.

      The caliper piston is 50mm and this is the MC I bought.
      Last edited by sam000lee; 06-22-2017, 10:04 AM.
      1980/1981 GS450 - GS500 Cylinder + Piston Swap - "De-L'ed", custom seat, CB350 bits, 18" rear, etc.
      1977 GS550
      1977 GS750 - Cross country trip thread

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        #33
        Okay so I did some math. My caliper to MC piston surface area ratio is:

        (pi x 25^2)/(pi x 7.9^2) = 10.0144

        which according to the chart on this thread:


        puts my brakes in the `wooden' category. Going down to a 14mm MC bore puts me at 12.8367 which is a more appropriate ratio....
        1980/1981 GS450 - GS500 Cylinder + Piston Swap - "De-L'ed", custom seat, CB350 bits, 18" rear, etc.
        1977 GS550
        1977 GS750 - Cross country trip thread

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          #34
          New pads are better than trying to clean old ones. I've tried "cleaning/sanding" just to get a bike to a trial ride but so many have had leaking fork seals before I got them, and I never noticed any improvement whatsoever. The stuff goes deep into the pad apparently. Alcohol and solvents just seem to make it worse. Id expect putting them in an oven would again, make it worse.

          Sanding them means that like new pads, they need a little break in to match the discs grooves. It may go some way to explain why pads are made the way they are...to wear in to these quickly.

          as to bore size, that's right. ....but If it feels okay at the lever, it is ok. Even if the pads are contaminated, you will feel that you are applying them hard.
          Last edited by Gorminrider; 06-22-2017, 10:34 AM.

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            #35
            From what Ive been able to find, most of the single disc use 14mm master cylinder and the duals use 5/8" (15.875mm).
            1982 GS1100E "Jolene"

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              #36
              Originally posted by TxGSrider View Post
              From what Ive been able to find, most of the single disc use 14mm master cylinder and the duals use 5/8" (15.875mm).

              I'm starting to think that maybe my MC was replaced at some point with one from a dual disc model.
              1980/1981 GS450 - GS500 Cylinder + Piston Swap - "De-L'ed", custom seat, CB350 bits, 18" rear, etc.
              1977 GS550
              1977 GS750 - Cross country trip thread

              Comment


                #37
                Also, its possible that it had the wrong master cylinder to begin with as you noted in an earlier that the rebuild kit that shouldve worked was too small and didn't fit.
                1982 GS1100E "Jolene"

                Comment


                  #38
                  Originally posted by sam000lee View Post
                  I'm starting to think that maybe my MC was replaced at some point with one from a dual disc model.
                  lol- I was posting that when you posted.

                  Maybe you can "borrow" the one off your 450 and see if it works.
                  1982 GS1100E "Jolene"

                  Comment


                    #39
                    Single front brake caliper GS bikes should be using a 14mm master cylinder. 5/8" masters are for dual caliper bikes. Using a 5/8" bore master on a single caliper system will result in a very firm lever feel, and require a very firm pull on the lever to generate brake force. Since your bike has a spongy lever that suggests there are other problems. Most likely the caliper piston is sludged up and/or the sliding pins are binding.

                    Standard protocol for reviving a 40 year old bike includes a full brake system teardown and rebuild.

                    -Remove the caliper pistons and clean out the grunge
                    -Replace the piston if pitted and install new OEM Suzuki piston seals
                    -Grease the sliding pins with heavy black brake caliper grease
                    -Replace pads as needed
                    -Replace brake line
                    -Remove the master cylinder plunger and clean out grunge
                    -Inspect master bore and replace if there is any pitting
                    -Dual caliper bikes take a 5/8" master and single caliper bikes use 14mm master
                    -Bleed the system after first priming the master by pulling vacuum on the bleed nipple

                    If the lever feels spongy after bleeding that means the caliper piston is binding and not moving close enough to the disc. Tying back the lever overnight after building pressure in the system usually helps in this regard. If aftermarket caliper piston seals are used all bets are off.
                    Ed

                    To measure is to know.

                    Mikuni O-ring Kits For Sale...https://www.thegsresources.com/_foru...ts#post1703182

                    Top Newbie Mistakes thread...http://www.thegsresources.com/_forum...d.php?t=171846

                    Carb rebuild tutorial...https://gsarchive.bwringer.com/mtsac...d_Tutorial.pdf

                    KZ750E Rebuild Thread...http://www.thegsresources.com/_forum...0-Resurrection

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                      #40
                      Originally posted by TxGSrider View Post
                      lol- I was posting that when you posted.

                      Maybe you can "borrow" the one off your 450 and see if it works.

                      I was actually considering just migrating the whole system over to avoid any bleeding so I could ride while waiting for parts. On my 450 the brakes are awesome.
                      1980/1981 GS450 - GS500 Cylinder + Piston Swap - "De-L'ed", custom seat, CB350 bits, 18" rear, etc.
                      1977 GS550
                      1977 GS750 - Cross country trip thread

                      Comment

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