Hard (power) brake pedal

Brakes for your Classic Mopar

  1. markfh

    markfh Well-Known Member

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    So got the steering box rebuilt and new exhaust. Boiled, soda blasted, and rebuilt the AVS and now I'm on the brakes.

    I replaced everything on the brake system except the power booster and am using synthetic brake fluid.

    It seems the power booster isn't boosting. The pedal is hard to the point I don't trust them.

    I've gotten lots of good advice here on other projects so I'm hoping y'all can help me with this.

    Thanks
     
  2. moparmat2000

    moparmat2000 FABO Gold Member FABO Gold Member

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    Booster prob shot. Unplug it, and vac it down with a vacuum pump. See if it holds vacuum. If not, time to replace rebuild it.
     
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    • SGBARRACUDA

      SGBARRACUDA ROY FABO Gold Member

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      What have you tried so far? Easy way to tell if the diaphragm is good, is start the engine, run it for just a short while, shut it off and unplug the valve from the brake booster, you should hear the vacuum release.
       
      Last edited by a moderator: Oct 15, 2019
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      • AJ/FormS

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

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        The following assumes that your hydraulic system is fully functional. If your caliper pistons are jammed tight to the rotors or the rear drums are over-adjusted, or Combination valve is stuck, or the Flex-lines are not passing fluid, or something is not plumbed correctly or the master cylinder is defective; then you gotta fix that first.
        Ok
        with the hose verified to be good, not plugged, and properly plumbed from intake manifold to vacuum chamber;
        engine off, evacuate the booster by pumping the pedal about 5 times. Now step on it moderately with something like 50 pounds of foot pressure, just guess. Then start her up and wait a couple of seconds; the pedal should fall about 1 to 1.5 inches, by itself. Maintain the pressure. If it doesn't drop, rev the engine up to something like 1200rpm, and wait a couple of seconds. If still no drop, relax and shut it off.
        Now, the diagnoses is that either the pushrod is too short or the booster is shot.
        Pop the hood, and at the booster,by hand, pry the checkvalve over to the side and listen for air rushing into the booster. You do have a checkvalve right?
        If no air rushes in, then put the valve back, start the engine, and let it idle. Get a helper to press the brake pedal down just a little. Grab a long nosed plier and squeeze the hose tightly shut, wait a second or two,and observe the engine idle quality. If the engine smooths out with the hose pinched, you can bet the engine is sucking air through either the diaphragm and/or the control valve, in either case the unit is no good.Some can be rebuilt. If the control valve is broken, it's probably because the pushrod is too long and someone accidentally hammered on the pedal too hard. You will have to investigate this before you install a new booster.
        But if air rushes in then both the diaphragm, and the control valve are likely Ok; leaving the pushrod adjustment.
        For a hard pedal the pushrod is too short. Just unbolt the booster(see edit1 below) from the studs and slide it towards the radiator just far enough to peek in there behind it. There you will see the pushrod with it's acorn-nut adjuster. Your job will be to make it one turn longer, but you have to anchor the rod, IIRC there are flats on it for a small wrench. IIRC that threading is "normal" so CCW from the engine side should do it.
        Now bolt it all back together, and retest.
        If still no drop, make the pushrod another turn longer and retest.
        Repeat until the nut falls off the end lol. If this happens to you; either the pushrod is wrong or the control valve is outta sight, or there is something wrong with the pedal system. I'll bet the booster is bad.
        The reason you only go one turn atta time is because if the control valve is ok, and if the pushrod gets to be too long, and if you mash the pedal down too hard; then it is possible to break the control valve and instantly you have ruined the booster. Don't let this happen to you.
        If/when you start to get boost,then you can fine tune the amount of boost playing with the pushrod length until you like the way it modulates.
        But if the booster goes from no boost to wham, then the control valve is for sure defective, and by definition, you need a replacement booster.
        Count the turns as you go, cuz if the booster gets replaced you will want to shorten the pushrod back to where you found it... at least.......... so you don't break the new one right away, on the first panic stop.
        That's my best shot

        Edit 1; unbolt the M/C from the booster; sorry.
         
        Last edited: Oct 17, 2019
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        • George Jets

          George Jets 1967 Dart 2 Door FABO Gold Member

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          Did you put on a new Master Cylinder? Proper one for Drum Drum, or Disc Drum. Power or Manual Master Cylinder ?

          If you have Drum Drum the master cylinder reservoirs should be the same size.

          What is the Bore of your master cylinder ? 15/16", 1", 1 1/16 ??? the larger the bore the harder the pedal will be as you have less leverage and less of a stroke making it harder to press.

          Did you add Front Disc Brakes, that is a whole other can of worms.

          More Info Needed . . .
           
        • markfh

          markfh Well-Known Member

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          Okay. After some exhaust issues with the head pipe flange leaking I'm back to the brakes.

          This is drum/drum. The check valve seems okay.
          "engine off, evacuate the booster by pumping the pedal about 5 times. ..... If still no drop, relax and shut it off."

          That's what I got.

          Won't have a partner to help until tomorrow.

          Here's a picture of the booster.

          dart booster.jpg
           
        • George Jets

          George Jets 1967 Dart 2 Door FABO Gold Member

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          Looks like the right master for drum drum.

          Like to go with simple basic things first. Jack the car up on jack stands, have a partner step on the brakes while you are spinning the wheels by hand. See how much brake pedal pressure it takes to actually stop the wheels from turning.

          Check the fronts and the rears. Generally the power master cylinders have the larger bores compared to the manual masters.

          See what you can figure out. Did you put new rubber brake hoses on ? They can collapse internally being 50 years old and you can't see that from the outside. Makes for one stiff pedal when the brake fluid can't get through the rubber brake hose.

          So here again, test spin and brake all 4 wheels to make sure the fluid is getting through.
           
          Last edited: Oct 16, 2019
        • markfh

          markfh Well-Known Member

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          I replaced everything in the brake system except the booster and the axle tee block.
           
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          • markfh

            markfh Well-Known Member

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            AJ

            "Just unbolt the booster from the studs and slide it towards the radiator just far enough to peek in there behind it. There you will see the pushrod with it's acorn-nut adjuster."

            Did this and didn't see the acorn-nut adjuster.

            Here's what I see.

            IMG_3707.jpg

            IMG_3708.jpg
             
          • AJ/FormS

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

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            Ohchit, I told you wrong.I have used that exact booster in the past.
            The adjuster is always between the booster and the M/C. Sorry for the aggravation.
            I edited post#4 for future readers.
             
            Last edited: Oct 17, 2019
          • markfh

            markfh Well-Known Member

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            No problem. That makes more sense. I'll check that tomorrow and work on it and let you know.
            Thanks
             
          • AJ/FormS

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

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            Sorry for your lost time and aggravation.
             
          • markfh

            markfh Well-Known Member

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            Time I have being retired and having been married for 47 years I've learned to deal with aggravation. :)
             
          • AJ/FormS

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

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            Tell me about it 42yrs for me.
            Can you imagine Adam and Eve........ I mean she screwed up Adams life right from the get-go :(
             
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            • markfh

              markfh Well-Known Member

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              Okay, made the rod a little longer but didn't change much and in the process discovered the check valve grommet is bad and leaking so after I get a new one in I'll keep testing.
               
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              • markfh

                markfh Well-Known Member

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                Finally got back to it on the brakes issue. When I depress the brake pedal the engine increases in rpm and shakes as you can see in this video.

                 
              • AJ/FormS

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

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                Whoaaaa, that is a serious air leak in the booster
                 
              • markfh

                markfh Well-Known Member

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                Know where I can get a rebuild kit? And how to tell which booster.
                 
                Last edited: Nov 2, 2019
              • SGBARRACUDA

                SGBARRACUDA ROY FABO Gold Member

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                Bad diaphragm in the booster.
                 
              • RealWing

                RealWing FABO Gold Member FABO Gold Member

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                Booster Dewey is highly recommended
                 
              • markfh

                markfh Well-Known Member

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                He'll get it tomorrow. :)
                 
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