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Herms Dual M/C Conversion

Discussion in 'Early CJ5 and CJ6 Tech' started by jhuey, Apr 2, 2012.

  1. Apr 2, 2012
    jhuey

    jhuey Michigan Jeeper!

    Indian River...
    Joined:
    Apr 9, 2004
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    415
    I have Herms dual master cylinder conversion on my CJ. I am having problems with the push-rod. Initially the travel of the push-rod would jam in the piston. I fixed that by bending the rod where it attaches to the pedal making it a straighter push. Well the rod broke next in the threaded adjustment area. I have had nothing but issues with the push-rod and any updated designs I have fabricated. The rod is snapping at the threaded area (weakest spot). I'm looking for any ideas you folks have come up with.
    [​IMG]
    When I originally installed it, this kept jamming the piston.
    I will try to find and post a picture of my most recent attempt at the push-rod design.
     
    Last edited: Apr 2, 2012
  2. Apr 2, 2012
    MitsJ54

    MitsJ54 New Member

    Joined:
    Jan 6, 2012
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    You might also contact Herm for discussion. I put one of these on my CJ5 a few years back. I found that I had to place a washer or two on the outside mc mounting bolt to tilt the mc correctly to avoid jaming and I never had a rod problem. The rod will not break unless it jams. You must have free travel of the rod. The conversion worked ok but I was never happy with the pedal pressure needed, which was too high. Last year I changed it out with the ABS Electric Power Brake which bolted directly onto the Herm bracket and provided power brakes. This has worked well, but it was not cheap.
     
  3. Apr 2, 2012
    jhuey

    jhuey Michigan Jeeper!

    Indian River...
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    Found a pic, not a good one.
    [​IMG]
     
  4. Apr 2, 2012
    barucker

    barucker Member

    St. Louis area
    Joined:
    Nov 10, 2006
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    I used a bracket from a '68 and had to fab part of a new pushrod using a bolt to get the length right. and I may also have hadto use the '68 pedal to get the right alignment. It seems the pivot sticking out of the pedal where the pushrod attaches was different/longer. Looks like the adaptation shown above works.
     
  5. Apr 3, 2012
    jhuey

    jhuey Michigan Jeeper!

    Indian River...
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    My push-rod is what is failing. I understand why but I don't know what else to use. The push-rod is snapping at the end of the threaded part (rod is weakened by the threading process).
    [​IMG]
    The last time I busted it I was pulling someone out of the mud and took a strain and was holding the brakes.
     
  6. Apr 3, 2012
    MitsJ54

    MitsJ54 New Member

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    I have some concerns with the shaft coming from the bottom of the pedal eye rather than pushing from the center.
     
  7. Apr 3, 2012
    jhuey

    jhuey Michigan Jeeper!

    Indian River...
    Joined:
    Apr 9, 2004
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    I am always concerned with any mods to the braking system, I had to do something to make the push-rod be more of a straight shot to the M/C. The drawing shows it at the bottom, it's not, it is about 1/3 of the way down from center. That part has been very strong and the failures (x3) have been on the threaded part of the rod, not to any other part. It has failed when EXTREME pressure has been put on the pedal, snapping the rod at the end of the threads. I'm thinking if I can find a 5/16 grade 8 long shouldered bolt with rolled threads insted of cut, and grind the hex off the end it might work. Next season I plan on a complete, long overdue brake upgrade, disc's all around, and a hanging pedal. I am not happy with the M/C down in the frame.

    I should of just shimmed the bracket when I started to have issues with the push-rod jamming, but I just didn't think of it when i was installing the dual M/C. I think my engineering changes are the issue, but I have to live with what I have now. It just seems so simple, but it's not. I guess if anyone has a stock link and push-rod I might be in the market. Than I would try the shimming process.
     
    Last edited: Apr 3, 2012
  8. Apr 3, 2012
    w3srl

    w3srl All-around swell dude Staff Member

    Port Orange, FL
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    I'm not sure that a Grade 8 bolt will do what you want, in fact it may be more prone to breaking. In order to get the tensile strength to meet Grade 8 specs the steel also gets brittle. The bolt is then very strong along its axis when put under tension, but in the process you lose a great deal of shear strength.

    I'm thinking that you need to either solve the offset issue or use a rod with more meat to handle the lateral stress.
     
  9. Apr 3, 2012
    MitsJ54

    MitsJ54 New Member

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    Failure will not occur with direct - straight pressure on the rod. It will only occur with twisting or binding. An off-set can cause this twist. You need to set it up so the rod applies pressure directly center of the pin. Check for any binding. If you must re-adjust the postion of the master cylinder, by tilting, or changing its mounting location, then you must do it or you will continue to have a problem where and when you cannot afford to have one. As you said, the problem will occur at the worst time, with hard pedal push.
     
  10. Apr 5, 2012
    barucker

    barucker Member

    St. Louis area
    Joined:
    Nov 10, 2006
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    You may have to put a new hole in the brake pedal and move the pin. you have to have a straight push, or maybe the assembly is to long?
     
  11. Apr 5, 2012
    jhuey

    jhuey Michigan Jeeper!

    Indian River...
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    I'm thinking I just have to start over. The existing engineering has stood the test of time, by changing what I thought was the right thing to do (using my logic) I moved a new issue into the mix. I will be looking for a new (old) push-rod assembly and tip the M/C to align up the rod.
     
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