Need some help with brake issue

Brakes for your Classic Mopar

  1. 71DodgeDemon340

    71DodgeDemon340 FABO Gold Member FABO Gold Member

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    My thinking is the pedal should feel how power brakes feel with the engine off and after the pedal has been pumped a few times until the vacuum is gone.
     
  2. 71DodgeDemon340

    71DodgeDemon340 FABO Gold Member FABO Gold Member

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    I appreciate all the help
     
  3. AJ/FormS

    AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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    That is sorta normal; during the first inch or so of pedal travel,the piston in the MC is moving past the C-port to close it off. If you whack it in that inch, yeah I can imagine a lil geyser but just a lil one is all I can imagine.
    Brake fluid does not begin moving until the C-ports are closed.

    Below is a picture to help you understand, Those blue springs are sitting in brake fluid and so is all the area in between them.
    As you push on the pedal, brake fluid first gushes into the rear chamber until the C-port is closed. Then the fluid trapped ahead of the rear piston hydraulically couples that rear piston to the front one, and begins pushing it past the front C-port. Now we are ready to start braking. The fluid in the front chamber now moves out, while the fluid in the chamber between the springs may or may not be moving out, but when the front piston encounters resistance, you keep on pushing down on the pedal. NOW for sure the chambered fluid starts moving out.
    That is how it is supposed to work.
    But if there is air trapped in the inter-piston chamber, then it just gets compressed. You still get some brake action, but nothing really happens until the front piston encounters resistance. By this time you may be pressing hard enough to drive the the front piston mechanically. Look inside the rear spring and see the pushrod in there. When this happens, the pedal is very low, and the air in there can be at a very high pressure; so when you release, it explodes back into the reservoir.
    So, the best way to set this up is, firstly without trapped air,lol, is so that the rear shoes start back-feeding into the MC as early as possible, so the front piston has only a limited range of motion. In that way, assuming no air in the MC, the rear piston can also get to work early, and so you will get a high hard pedal.
    But as you can see, air in the inter-piston chamber just kills it.
    Hope this helps.
    upload_2021-3-21_17-24-25.jpeg
     
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    • 71DodgeDemon340

      71DodgeDemon340 FABO Gold Member FABO Gold Member

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      Thank you that makes a lot more sense, with the cap off the fluid in the rear drum chamber geysers up about 2” above the master when i first hit the brakes, only did that once luckily the fluid came back down in the master
       
    • AJ/FormS

      AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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      yeah it can be tricky and messy. There are two ways for air to get out of the MC. You can tickle it out by dropping the front end of the MC and having it escape thru the C-ports.
      Or you can crack the lines one atta time and push it out. But when you do that, you gotta slam the door before letting off the pedal, or the returning piston will just suck air at the fitting.
      But take another look at the drawing. If your MC is upward slanting as they usually are, then you can drive the inter-piston fluid and air into the front reservoir thru the compensating port, by cracking the fitting at the front reservoir, badaboom!
       
    • 71DodgeDemon340

      71DodgeDemon340 FABO Gold Member FABO Gold Member

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      I just thought, if the drums are out of spec could that cause issues with the wheel cylinders having to move further or would it not really matter? What is the discard measurement on the 10x1.75 drums? I have the original bell style ones now
       
    • AJ/FormS

      AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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      The self-adjusting slack adjusters are supposed to take care of that, but if they quit working, and the pistons go out without pushing the shoes into the drums; then the piston in the MC will bottom out. When that happens, with NO air in the MC , and with a properly functioning front brake system,then the pedal will get very low, but the front brakes will still work normally.
      In fact, I have seen many cars come into the shop functioning this way, and the only complaint was the low pedal.

      IDK about the discard measurement. I'd guess maybe .090. But the max ID is written on the drum somewhere. or should be. However, if you machine the drums, you really need to get your new shoes sized to fit them, else the shoes will;
      1) initially, only contact in a very small area, the areas growing larger with time, unless
      2) they overheat and glaze up, and possible crack. and
      3) the self adjusters will have to work overtime trying to keep up with the rapid wear during the extended break-in period.
      D) but if the self-adjusters quit, then your pedal is gonna rapidly stop closer and closer to the floor,
       
      Last edited: Mar 22, 2021
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      • 71DodgeDemon340

        71DodgeDemon340 FABO Gold Member FABO Gold Member

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        reason i ask is because i measured mine at like 1/16 to 1/8 over 10” which would put me right at the discard spec assuming .090 over 10”. Also i noticed my shoes werent wearing as much at the top and bottom of the shoe. Theres about a inch on the top and bottom of the shoes where it doesnt really look like they have wore. Of course the car may get driven 15-20 miles a week or every two weeks so not a lot of braking going on
         
      • 71DodgeDemon340

        71DodgeDemon340 FABO Gold Member FABO Gold Member

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        I ordered new wheel cylinders and shoes from napa today, will take a few days to get them, i still need to order new hardware and thinking about new drums as well
         
      • AJ/FormS

        AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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        That would be normal in your situation where the drums are so worn. Your shoes are finally almost broke in, to these drums.
        But that is in fact abnormal wear; those drums should be changed or; should be machined if still allowable, AND custom shoes made for the large ID drums.
         
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        • 71DodgeDemon340

          71DodgeDemon340 FABO Gold Member FABO Gold Member

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          also they were about 6-8 clicks out of adjustment but not sure if that was enough to cause them to wear like that, i always use the parking brake so i figured they would stay adjusted
           
        • AJ/FormS

          AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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          Nope that's got nothing to do with it. That wear pattern is ALL on the mismatch between the drums and the shoes.

          and nope. the parking brake has very little to do with adjusting. The system only works when backing up, at the application of the brake pedal. As the shoes, all tied together with the springs and such, counter rotate in reverse, the lil cable operated adjuster tries to add one click of adjustment, every time you tap and release the brakes. When the adjustment is completed in this manner, the cable deal runs out of adjusting range and no more clicks will be added until the shoes wear enough to allow another clic.
          Once adjusted the Parking brake cannot make the system work.
          But if the system was grossly out of adjustment, I suppose the parking brake could pull in a couple of clics, but again, I think the car would have to be backing up. Meh, maybe not.

          That sounds sorta normal, if you mean 6 to 8 from as far as it goes. But if 6 to 8 from just barely making the shoes be dragging then that would have been a LITTLE on the loose side, not too big a deal.
           
          Last edited: Mar 22, 2021
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          • 71DodgeDemon340

            71DodgeDemon340 FABO Gold Member FABO Gold Member

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            awesome thanks for the info! Learn something new everyday
             
          • AJ/FormS

            AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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            when you get your already ordered shoes, drop one of them into the worn drum, on the worn surface, grab one end and pinch it tight to the drum; then observe how far the other end is from touching. Those shoes, in that worn drum, will not be doing much of anything, until a great deal of the surface gets rubbed off. And during the rubbing, it will get very very hot, because it is only touching on a fraction of the drum, which is supposed to be it's heat-sink. Don't be surprised to be smelling it for a long time, every time you exit the vehicle.
            Or, you can take your drums and shoes, down to the brake-shop, and have them matched.
            But, I gotta tell-ya, the rear brakes are designed to do no more than about 15% of the total braking anyway. So if yours start out at 1% or 2%, is that a big deal?
            Only if they catch fire,lol.
             
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            • 71DodgeDemon340

              71DodgeDemon340 FABO Gold Member FABO Gold Member

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              Just a update, been busy doing the water pump after i did the brakes. Bled them again with the vacuum bleeder and double checked the rear adjustment. Pedal still felt a little low to me but seems like it was pretty firm, ordered a adjustable brake pedal rod and brought the pedal itself up maybe 3/8 to 1/2”. Pedal was firm a little higher but not like rock solid. So the crazy part is once i started the car the pedal immediately felt better like it should? Lol idk maybe it was just happy, took it for a short drive and all seems well! No leaks stops great. Does pull slightly to the right while braking hard but i could not rule out the road or tire pressure. Only have about 15 miles on the brakes with a handful of stops so need more time for the pads and sjoes to wear in
               
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              • kursplat

                kursplat FABO Gold Member FABO Gold Member

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                next time just bed those puppies in at the beginning.
                 
              • 71DodgeDemon340

                71DodgeDemon340 FABO Gold Member FABO Gold Member

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                the brake pads i put on have a titanium coating that claims the pads are bedded in in about 3 or 4 stops
                 
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                • 71DodgeDemon340

                  71DodgeDemon340 FABO Gold Member FABO Gold Member

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                  wether or not it actually is true im not sure lol
                   
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