GS400 Front Brake Caliper Piston Size? And question?

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  • NewRider
    • Feb 2026
    #1

    GS400 Front Brake Caliper Piston Size? And question?

    Hey Guys,


    I have a question I was I was hoping you could answer-- I recently purchased a replacement M/C brake lever unit with a 14mm piston for my 78' GS400. After purchasing I stumbled upon this http://vintagebrake.com/mastercylinder.htm

    I believe the stock M/C has a 13mm piston and the stock front caliper in a GS400 has a 38mm piston. If someone could confirm this I would appreciate this because I really can't find a definitive source. If the article is correct and my assumption about the gs400 piston sizes is correct, will my braking be horrible?


    And if so can you change the caliper piston size without replacing the caliper?


    I'd like to get my ratio to ~12 as mentioned by the author of that link.
    Tags: brake calipers, caliper, front brake, master cylinder, piston
    • Guest
      #2
      Your shop manual will have all the piston sizes you are looking for.


      Mark

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        • NewRider
          #3
          Originally posted by mmattockx
          Your shop manual will have all the piston sizes you are looking for.


          Mark
          The Clymers Manual doesn't give the Piston Sizes for either the M/C or Caliper I just double checked. Unless it's listed somewhere outside the brake section.

          Edit: I might have missed something but I just went through the entire manual and didn't find them listed anywhere.
          Last edited by Guest; 08-14-2014, 07:53 PM.

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            • posplayr
              Forum LongTimer
              GSResource Superstar
              Past Site Supporter
              • Dec 2007
              • 23673
              • Tucson Az
              #4
              Originally posted by NewRider
              The Clymers Manual doesn't give the Piston Sizes for either the M/C or Caliper I just double checked. Unless it's listed somewhere outside the brake section.

              Edit: I might have missed something but I just went through the entire manual and didn't find them listed anywhere.
              around here Clymer doesn't print Manuals. There is a company that does . Names starts with an "S"

                Comment

                • NewRider
                  #5
                  POS I wish I had the Suzuki Manual. When my Dad gave me his GS he no longer had it.... I'll have to check Ebay. In the mean time I'm working with the Clymers from bikecliffs website-- I'm a college student with a tight budget so free is better than $.

                    Comment

                    • Guest
                      #6
                      I missed these earlier...

                      Originally posted by NewRider
                      If the article is correct and my assumption about the gs400 piston sizes is correct, will my braking be horrible?
                      Your braking effort will go up by a factor of (14/13)^2 = 1.16 or 16% more than before. Whether that is horrible or not I can't say.


                      Originally posted by NewRider
                      And if so can you change the caliper piston size without replacing the caliper?
                      Nope, the piston size is part of the caliper body geometry. If you want bigger pistons you would need to find a different caliper.


                      Bikeclilff's site doesn't have the service manual for your bike?


                      Mark

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                        • NewRider
                          #7
                          Thanks Mark and negative. Bikecliff's only has the Clymers for the GS400

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