1. mrbone

    mrbone Well-Known Member

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    I have a 1968 roadrunner with newly serviced manual drum brakes front & rear.

    SYMPTON: Brake pedal is 100% when pumped ounce then applied. But when brake pedal pressure is reduced then applied again, pedal goes to the floor.

    The brakes on my first roadrunner I owned in high school would 'fade' the same way.
    So I haven't given it much thought.

    If it is "normal", is there anything I can do to remedy this?
    If not, any ideas what's going on? Seems like bad master cylinder. Wonder if both cars coincidentally had the same problem?

    What do you guys think?

    THANKS!! AJ
     
  2. davek

    davek FBBO Gold Member FBBO Gold Member

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    pedal going to the floor is not normal. any residue around the top of Master? Have you bleed the lines?
     
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    • 69L48Z27

      69L48Z27 Well-Known Member

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      Plug the ports on the MC. If you get the pedal to the floor then the MC’s got some issues. Other than that bleed everything again and make sure you have everything adjusted up.
       
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      • mrbone

        mrbone Well-Known Member

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        OK! That's what I needed to know.
        I will block the ports on the MC when I bleed the brakes switching over to DOT5 fluid.
        Waiting for new lines to replace the rubber one's on the rear, so when I get them I will start.
         
      • coronet68mx

        coronet68mx FBBO Gold Member FBBO Gold Member

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        A lot of guys here recommend gravity bleeding procedure for best results.
         
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        • 68Moparmaniac

          68Moparmaniac Well-Known Member

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          Not sure about DOT5 in your system. Maybe switch to DOT3 for the older drum system...
           
        • 493 Mike

          493 Mike FBBO Gold Member FBBO Gold Member

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          DOT 5 will be fine IF the entire system is clean/devoid of old fluid.
          Mike
           
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          • Demonic

            Demonic Well-Known Member

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            A long time ago, I read in one of the magazines to flush the system with denatured alcohol when switching to DOT 5. I ran DOT 5 in my Demon years ago - the pedal was firmer with DOT3.
             
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            • mrbone

              mrbone Well-Known Member

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              Any favorite master cylinders out there for disk/drum?

              Looks like Doc Diff has a nice one.
              15/16, aluminum and uses factory pushrod..
               
              Last edited: Feb 21, 2020
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              • davek

                davek FBBO Gold Member FBBO Gold Member

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                I dont think you can go wrong with Dr Diff,I bought one from summit with the 15/16 size. I googled reviews on the MC, You could look at mancini racing to see what they have.
                 
              • beanhead

                beanhead Well-Known Member

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                I like the 15/16" manual setup. However, keeping the rear drums adjusted properly is crucial to proper pedal operation. I think that was mentioned in one of the posts above. With either system, if drums are on the car they must be adjusted correctly. Super easy to check and fix, always the first place I start if having pedal issues.
                 
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                • Rusty 72

                  Rusty 72 Well-Known Member

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                  Yep I agree its not normal. You should have a firm pedal at all times. If its not the MC, then I would go through the entire system. You could have a weeping wheel cylinder or a leak somewhere in the lines. A small rusty spot in a line will become a big problem when it gives out entirely.
                  Good luck!
                   
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