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  • front caliper locks up

    got a GMA 2-piston front caliper that will lock up and stay there until i bleed it out. so i've been bleeding it and leaving the bleeder open while prying the pads back into the case. then close the bleeder and have no brakes up front until they get pumped back up at which point they will again lock up. the pistons are NOT retracting into the case for some reason; i think there may have been a cross contamination of different fluids. is there a way to just flush the whole system with something to neutralize it then start over with the proper fluid?? or do i have to disassemble everything and clean it to the bone??
    I love my freedom so i thank GOD and veterans, not politicians!! sweet home Arizona

  • #2
    if you do a flush
    use denatured alcohol
    Last edited by jsoko; 10-23-2019, 06:52 AM.
    Huntin' Fishin' And Lovin' Every Day
    Northwest Georgia
    1954FL ~ 1978FLH ~ 1997FXD

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    • #3
      If the pistons don't return unless you crack the bleeder, the most likely cause would be a plugged compensating port, or the piston is not returning. I'd be looking at the master cylinder, not the fluid. Fix it first or you'll be wasting fluid.
      The primary and the secondary are on the same side.
      78 FXS 74" 4 speed, 79 FXE basket

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      • #4
        You may save yourself from some grief by just going ahead and disassembling it for a good cleaning. If you find crap in the piston's bore, look at the brake hose. They do break down and release rubber boogers when they get old.
        Not what you expect from this forum
        Forum fucktard
        Winner of the Asshole thread...

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        • #5
          What Bone said...and when you pull the piston out of the bore look at it and the piston carefully. It can be rusted or worn and if it is a new piston may be needed. If it is bad the caliper may need to be replaced. Also pull the master cylinder apart and observe the bore and piston for the same...
          My Website: www.amcpchoppers.com

          sigpic:

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          • #6
            ''Also pull the master cylinder apart and observe the bore and piston for the same''..

            It does sound more like your MC is holding pressure more than the pads or caliper doing anything funny.

            IF, the wrong fluid was used I could see that affecting the MC quite quickly, it's like trying to use jello for fluid. This could most definitely clog the little hole in the MC and cause it to not drop back down in pressure.

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            • #7
              Another vote for the master cylinder as the problem.
              Mike
              Central Alabama

              sigpic"Be who you are and say what you feel, because those who mind don't matter, and those who matter don't mind."

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              • #8
                Probably MC as stated... But, Since I am a COB, I have had old crappy brake lines develop a "flap" or "valve" into them and give exactly the symptoms described...
                Believe It or Not!!!!
                Oh, and if the Brake line is "New".... I double suspect the damn thing.... I prefer Tested/Working

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                • #9
                  Originally posted by mike the snake View Post
                  ''Also pull the master cylinder apart and observe the bore and piston for the same''..

                  It does sound more like your MC is holding pressure more than the pads or caliper doing anything funny.

                  IF, the wrong fluid was used I could see that affecting the MC quite quickly, it's like trying to use jello for fluid. This could most definitely clog the little hole in the MC and cause it to not drop back down in pressure.
                  this sounds like the nail on the head. although the MC is near new. i think the fluid crossing could just as easy toast the new MC as and old one....thanks for all the ideas.....i will tear it done to the bone then maybe some isopropyl alcohol. then the correct fluid which is compatible with all other types.....the only one!!!!
                  I love my freedom so i thank GOD and veterans, not politicians!! sweet home Arizona

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                  • #10
                    i will tear it done to the bone then maybe some isopropyl alcohol.
                    if you are going to Flush
                    isopropyl alcohol NO = attracts and can contain moisture
                    denatured alcohol YES = no moisture, prefered
                    Last edited by jsoko; 10-23-2019, 11:44 AM.
                    Huntin' Fishin' And Lovin' Every Day
                    Northwest Georgia
                    1954FL ~ 1978FLH ~ 1997FXD

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                    • #11
                      Yeah if the brake fluids have been mixed (dot 4 and dot 5) it turns it all to jello and jelly like goo. Denatured alcohol and a SERIOUS flushing and cleaning (IMO INCLUDING disassembly of the caliper and MC and clean until it looks like new) will get you back to a starting point.

                      Then, it's just me, but I stay away from silicon Dot5 at all costs (unless it's already in a good functioning setup and confirmed as Dot5) I guess over 30 years of racing and wrenching and doing brakes just has me unconsciously grabbing for dot3 or dot4.

                      My first Harley had this mistake mix in my rear juice drum, wouldn't retract the pads, and it took complete disassembly to clean thoroughly-Everything!. It is a nightmare.

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                      • #12
                        Originally posted by jsoko View Post

                        if you are going to Flush
                        isopropyl alcohol NO = attracts and can contain moisture
                        denatured alcohol YES = no moisture, prefered
                        yes, thanks for reminding me.
                        I love my freedom so i thank GOD and veterans, not politicians!! sweet home Arizona

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                        • #13
                          Originally posted by mike the snake View Post
                          Yeah if the brake fluids have been mixed (dot 4 and dot 5) it turns it all to jello and jelly like goo. Denatured alcohol and a SERIOUS flushing and cleaning (IMO INCLUDING disassembly of the caliper and MC and clean until it looks like new) will get you back to a starting point.

                          Then, it's just me, but I stay away from silicon Dot5 at all costs (unless it's already in a good functioning setup and confirmed as Dot5) I guess over 30 years of racing and wrenching and doing brakes just has me unconsciously grabbing for dot3 or dot4.

                          My first Harley had this mistake mix in my rear juice drum, wouldn't retract the pads, and it took complete disassembly to clean thoroughly-Everything!. It is a nightmare.
                          While I stick to dot-5...as I like my Paint just like it is...and I am Sloppy!!!!
                          Besides...most of the iron I get to touch...came speced at dot-5... says so right on the OE Handlebars....LOL

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                          • #14
                            Folks around here have had line porblems

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                            • #15
                              One more happened to me.1978 dual disk stock Harley front end. Doing exactly as you described. Believe master cylinder was too small to flush air out of vertical lines. Back flushed from caliper to master cylinder. Problem fixed.

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