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Geo Disks With Stock Single Master Cyl

Discussion in 'Early CJ-5 and CJ-6 Tech' started by Jrobz23, Jun 1, 2019.

  1. Jun 1, 2019
    Jrobz23

    Jrobz23 Member

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    I've heard conflicting opinions. Has anyone done Geo Disks in the front and rear and run them off the stock single master?
     
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  2. Jun 1, 2019
    timgr

    timgr Jeepin' Nerd Sponsor 2019 Sponsor

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    It should work with disks front and rear. You would need proportionally sized pistons for front and rear, to balance the braking force.

    Most would convert to a dual cylinder simply because it's safer.
     
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  3. Jun 1, 2019
    colojeepguy

    colojeepguy Colorado Springs Sponsor 2019 Sponsor

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    This x1000
    As someone who has suffered total brake failure twice on vehicles with a single bowl master cylinder, I will not run one again.
    Sorry for the hijack, but I feel pretty strongly about this topic.
     
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  4. Jun 1, 2019
    Jrobz23

    Jrobz23 Member

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    I've lost a brake circuit (wheel cyl popped on pass rear drum) on my TJ and it was complete brake failure, pedal -> floor with no effect. In light of that XP, I don't put much stock in the dual masters. I do, however, put great stock in brake system maintenance.

    Also FYI, I do have a dual master on my '57. I'm not a fan of how it turned out fitment-wise, so I'm investigating options for keeping the single.
     
  5. Jun 1, 2019
    Jrobz23

    Jrobz23 Member

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    Come to think of it, I had the same thing happen on my Dodge this spring (except driver side rear wheel cyl/brake line). Thankfully I was parked. Complete brake failure. The TJ was at least a manual, so I could drive it home clutch braking.

    Insert joke here about my "great stock in brake system maintenance"
     
    Last edited: Jun 1, 2019
  6. Jun 2, 2019
    GTS Dean

    GTS Dean New Member

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    I am assuming that the two dual MC failures may not have installed a safety valve between the MC and the wheel positions. With a low-mount system like our Jeeps, it makes for a bit more complex plumbing and bleeding, but they protect against just the sort of issues mentioned.

    To briefly describe theory and funciton:
    It is a balanced pressure 2 circuit spool valve. Each side of the MC feeds into each end of the spool valve and out to the 2 axle circuits. Almost all of them now include a proportioning valve in the rear circuit and they are called combination valves. In the event of fluid loss in either circuit, the MC will feed all of one side of the reservoir until empty. This causes a pressure imbalance on one side of the safety valve, with the good circuit forcing the spool sideways to plug the leaking circuit and maintaining brake function on the other. There is a normally open switch in the middle of the valve. When a failure occurs, the spool moving sideways will ground the circuit and illuminate the brake fault light.

    Most modern master cylinders are sloped so that the high side feeds the rear circuit. If fluid is lost, the rear circuit will lose function first. This is typically safer than having rears that may lock and cause a spin in corners.
     
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  7. Jun 2, 2019
    Jrobz23

    Jrobz23 Member

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    So back to the orig question. Has anyone done this?
     
  8. Jun 2, 2019
    GTS Dean

    GTS Dean New Member

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    I think it could be done, but I'd DEFINITELY install a safety valve first, and an adjustable prop valve in the rear circuit. The front calipers are going to require more fluid volume to operate and account for pad wear, so do some more investigation along those lines.
     
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  9. Jun 2, 2019
    Jrobz23

    Jrobz23 Member

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    Sound advice. Thanks for the good info!
     
  10. Jun 2, 2019
    Jrobz23

    Jrobz23 Member

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    GTS Dean, do you have a part# you recommend for the safety valve?
     
  11. Jun 2, 2019
    timgr

    timgr Jeepin' Nerd Sponsor 2019 Sponsor

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    I wonder about this. My understanding was there is no direct connection between the front and the rear halves of the master cylinder, other than the single piston that compresses both.The result of a leak would be determined by the master cylinder design, not by the "safety valve" between the circuits.

    How you could you install a spool valve (like the modern combination valves) in a single cylinder system? As described by GTS Dean, it depends on the pressure imbalance between the two cylinders to function. If you look at the Jeep TSM picture, this part of the combination valve is shown as the piston that activates the brake warning switch. Possible there is a "safety valve" specifically intended to stop single cylinder systems from draining when the front or rear circuit is opened, but I suspect not. The advent of dual master cylinders really limits the market for such a device.
     
  12. Jun 2, 2019
    timgr

    timgr Jeepin' Nerd Sponsor 2019 Sponsor

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    You can see how this works. The pin between the cylinders, or protruding from the secondary piston, limits the loss of pedal when one of the circuits goes soft.

    cylinder.png

    I experienced this when the rear brake tube on my J10 rusted through. I still had brakes, but about half the pedal height.
     
  13. Jun 2, 2019
    GTS Dean

    GTS Dean New Member

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    This "distribution block" looks like what you need - notice the contact switch. It would provide F/R circuit protection on a single reservoir system *ONLY IF* you installed a Tee between the MC and the block inlets. If you break a line at either end of the vehicle, the spool would displace and the other circuit should still work. I would imagine that the auto manufacturers and DOT debated the merits of dual MC vs single with tee to safety valve in the mid-60's before the split dual MC's came out around '67 or '68 for all auto manufacturers.

    This block valve (PV 05 or 06) will also require a separate proportioning valve for the rear system. I doubt that it makes any difference whether the inlet side is bottom or top. Lots of options at Summit Racing:

    Right Stuff Detailing Brake Proportioning Valves and Blocks PV05
     
    Last edited: Jun 2, 2019
  14. Jun 2, 2019
    45es

    45es Sponsor Sponsor

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    Your explanation leaves quite a bit out. Based on what you said, what occurs when there is a front brake system failure? The differential switch within the combination valve certainly does not lock out the rear brakes but it does alert you to a brake system failure.
     
  15. Jun 2, 2019
    GTS Dean

    GTS Dean New Member

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    Well, it does leave things a bit murky. In any event - on a dual MC one half of the system is still getting full MC pressure, regardless of which side of the MC it's coming from. That circuit would displace the spool and close the broken side, leaving the intact system working. If the failure is between the MC and the safety valve on a single reservoir system, you're SOL either way.
     
    Last edited: Jun 2, 2019
  16. Jun 2, 2019
    timgr

    timgr Jeepin' Nerd Sponsor 2019 Sponsor

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    Skeptical. Looking at the illustration in '76 TSM and reading the text, that sort of function is not supported. Look here -

    combovalve.png

    There is a seal that blocks fluid from passing by either side of the center piston. An brake fluid would come out the switch terminal if it did. It does not in my experience. And these switch terminals are plastic and could not stand full brake pressure.
     
  17. Jun 2, 2019
    GTS Dean

    GTS Dean New Member

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    We can debate the intricacies all day. I'm just trying to keep it simple.

    If the rear circuit fails in your diagram, the center spool (switch piston) will be forced to the right and the proportioning valve stem will be blocked shut. If the front circuit fails, the spool will move left and block the feed from the MC to the metering valve.

    Give it some more thought and hopefully, you'll come around.
     
    Last edited: Jun 2, 2019
  18. Jun 2, 2019
    timgr

    timgr Jeepin' Nerd Sponsor 2019 Sponsor

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    Possible. I can't tell enough about the proportioning section from the drawing to tell if the rear inlet port will be blocked, but movement to the front looks like it could block the metering valve. The text of the TSM says nothing about this, only attributing the warning switch function to the center valve. In my experience with such a failure on my J10, equipped with this combo valve, the failing side was drained dry.

    Regardless, how does this help a single cylinder master system? Or have we discarded that idea?
     
  19. Jun 2, 2019
    GTS Dean

    GTS Dean New Member

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    Nope - Post #15.
     
  20. Jun 2, 2019
    timgr

    timgr Jeepin' Nerd Sponsor 2019 Sponsor

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    Not sure what this reply means.

    Regarding the "blocking" function, trying to be pleasant and respectful, but I still don't see it. With a single cylinder system feeding both inlets, the piston would never activate to block one side or the other. If one circuit fails, all the fluid volume exits that side and there would never be a pressure difference across the piston. In a dual cylinder system, the only advantage of the "blocking" function is to keep from draining the failed circuit. Though this is an advantage, it seems kinda incidental, hardly a "safety" valve. The design of the master cylinder still pressurizes the intact circuit, regardless of whether the failed side drains or not. The failed side has failed, whether it's spurting fluid out a wheel cylinder or broken line, or bypassing the primary seal in the master.
     
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