1. Xtreamist

    Xtreamist Well-Known Member

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    My 67 Barracuda currently has drum brakes all around. I need a new master cylinder and I know the disk/drum MC is different than the drum/drum. From all I can tell is the disk/drum MC has a larger front res. My question is, will the disk/drum MC work for my current drum/drum brakes? I mean why would having more fluid in one res. hurt anything? At some point I will be doing the front disk brake conversion so I'll need that MC anyway. Also, would I need to install the prorp valve now or can that wait until I do the conversion?

    Thanks FABO....
     
  2. dartfreak75

    dartfreak75 Restore it, Dont part it! FABO Gold Member

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    I may get corrected but in my opinion if you are gonna run drum/drums you need a mc for that application! If you plan on doing a front disk swap anyway get the disk drum mc and try it out.
     
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    • Syleng1

      Syleng1 Karma is real and Life is short... FABO Gold Member

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      Since I just converted my 66 Barracuda from 9” drums all four corners to the 73-76 bbp 10” disks and 10x2-1/2” rears- the brake master cylinder for that has the larger chamber up front for more volume. When you step of the pedal the internal piston is larger to accommodate the caliper volume. Vs. the rear wheel cylinders are 60% smaller more fluid would bottom out the pedal on a single line system.

      I’m my opinion I would say “yes” you will have an issue with a dual in equal system until you actually have the components on there with the needed ratios to match the resivoir.
       
    • Donnie514

      Donnie514 FABO Gold Member FABO Gold Member

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      The two master cylinders are NOT interchangeable !!
      The size of the reservoir is larger for disc because as the disc pads wear the caliper piston does not return all the way back into the caliper. No return springs. The brake fluid stays in the caliper.
      The real difference in the two master cylinders is the check valves in the master cylinder. Drum brakes, with the check valve in the master cylinder all ways have a little pressure in the lines and wheel cylinders to keep the brake shoes cocked.
      Disc brakes have no pressure in the lines or calipers so they can just relax.
      If interested I have a NEW power brake set up complete, that should fit your car. Bolt on. For drum brakes.
       
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      • Xtreamist

        Xtreamist Well-Known Member

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        Well that does shed some light on my question. The MC's are cheap enough that when the time comes to do the conversion I will by the proper MC. Since I currently have drum/drum, I'll get the MC for that application. I was simply thinking ahead for the build. I sure would hate to have faulty brakes with the wrong MC for the set-up now.
         
      • Xtreamist

        Xtreamist Well-Known Member

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        Thanks for that information. I didn't know the internals were different, although I had a sense they were. I'm leaving the car manual brakes. Only doing a front disk conversion, at a much later time. Just need the drum/drum brakes to stop the car, VERY IMPORTANT!
         
      • Syleng1

        Syleng1 Karma is real and Life is short... FABO Gold Member

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        I think it would work but the balance may be a tad off. It is easier and honestly cheaper in the long run to just buy the right part for now and keep moving forward.
        Joe
         
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        • Mattax

          Mattax Just the facts, ma'am FABO Gold Member

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          The correct MC is actually pretty expensive at the moment. Regardless of the photo, most production rebuilders have been simply substituting using drum/drum MC's with no residual valve in the rear outlet for the past 20 years. :( It will work OK, but requires more frequent checking of the fluid level.

          Good reading on the system here: 1967 Chrysler Hydraulic Brake Master Tech - Session 232

          and here; 1967 Chrysler Disc Brake System Master Tech- Session 233
           
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          • Murray

            Murray FABO Gold Member FABO Gold Member

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            Donnie is correct, disc brakes need a larger reservoir due to the volume of fluid needed. Internals (residual check valves are different between disc/drum ) are different, so theoretically a disc brake M/C with larger reservoirs can also work on drum brake set-up, but not a drum brake M/C on a disc set-up (because of the smaller reservoir).
             
          • Dana67Dart

            Dana67Dart Like a fine wine, only getting better with age! FABO Gold Member

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            I love those booklets, they explain things to a level even I can understand!
             
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            • Mattax

              Mattax Just the facts, ma'am FABO Gold Member

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              Actually its done all the time. As long as the piston stroke and diameter are close, all they do is leave the residual valve out of the primary port. Not ideal, but safe enough the big companies have been selling that way for years.
               
            • Dana67Dart

              Dana67Dart Like a fine wine, only getting better with age! FABO Gold Member

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              Speaking of 67 master cylinders... is there supposed to be a bellows type boot between pushrod and firewall like in photo?

              upload_2019-3-30_10-26-28.png
               
            • RedFish

              RedFish Well-Known Member

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              Yes there should be some sort of weather boot at the firewall. Varies with model. Some have a boot bonded to larger rectangular plate that attaches at the perimeter bolts/studs. May find oval hole in firewall too. Rotten rubber is the only constant.
              Nearly 40 years ago, When I changed our first 67 B'cuda front brakes from 3X9 drums to 73 model discs I continued to use the original drum drum master until it failed, about a year later. Then a disc drum master went in. I don't recall ever installing a proportion valve in that car.
               
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              • Xtreamist

                Xtreamist Well-Known Member

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              • Mattax

                Mattax Just the facts, ma'am FABO Gold Member

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                My comment "The correct MC is actually pretty expensive at the moment." was in reference to the disk/drum master.
                 
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                • BillGrissom

                  BillGrissom Well-Known Member

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                  As mentioned a disk/drum MC would work fine. They don't come w/ the brake rod. You will need a new rubber bushing for the rod tip to lock in the MC piston. Insure the dust bellows is already on since you only get one chance. But, I wouldn't fool with an old cast-iron MC unless you like the rusty look and leaking brake fluid, i.e. paint remover. Use a later 2-bolt MC on a 2-4 adapter plate. Many use one for a 1980's Dodge truck. Dr Diff and others sell them kitted, or you can buy just the plate on ebay ($30). I used an MC for a 95 Breeze ABS w/ adapter on my 64 Valiant and fit fine, but there are a gazillion alum 2-bolt MC's out there. Of course they are for front disks since that was standard 1973+. There is a single reservoir w/ 2 chambers. The lid seals tight so no fluid loss or moisture getting absorbed. You can even wire the low-reservoir switch to gnd, in parallel w/ your e-brake switch (and imbalance switch for cars w/ those, ~1969+). Any that switch to ground will light the dash lamp.
                   
                • Dale Coulter

                  Dale Coulter 67 SuperFish

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                  yes that is correct. I did the same thing and it worked fine.
                   
                • DragginSteel

                  DragginSteel Active Member

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                  Does anyone know the nut size of the bolt(s) (4) holding the master cylinder to the pedal bracket?
                   
                • Donnie514

                  Donnie514 FABO Gold Member FABO Gold Member

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                  5/16 NC
                   
                • DragginSteel

                  DragginSteel Active Member

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                  Thank you! I guess it's time to get out the wire wheel, they don't even want to start with a 5/16 (it's good news though if it was a 3/8ths I was in trouble)
                   
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