Over sensitive brakes

Brakes, Steering & Suspension

  1. shovelluv

    shovelluv Well-Known Member

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    Is it possible a booster can give to much assist? I picked up an original low mile 1970 Newport convertible, 4 wheel drum power assist. I'm having a hard time believing this car was engineered to put you threw the windshield when you apply the brakes, it's terrible! I installed a new master for 4 drum power brakes because the old leaky one was for a disc/drum. Checked all 4 brake/drum assemblies behind the wheels and the primary shoe is facing the front of the car. Drums, shoes and hardware look great. Checked the gap between the master and booster and its .035. Any possibility a booster can be bad that it is giving too much assist? There is about a 1/2" of pedal push as its running before the booster kicks in and locks em up. Any help would be great!!
     
  2. 66Satellite47

    66Satellite47 FBBO Gold Member FBBO Gold Member

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    I would suspect contamination on the shoes. Could be rust, brake fluid or grease/lube. The adjusters could also be too tight. ???
     
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    • shovelluv

      shovelluv Well-Known Member

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      Thanks for the reply. The shoes are nice and clean on all corners and I checked the adjustment and auto adjusters. There is just a slight drag on the shoes to drum. Thanks
       
    • 69b3rt

      69b3rt Well-Known Member

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      My dads 69 sport fury is the same way with power drum brakes. It takes awhile to get used to the power assist. My mom hated driving the car.
       
    • 1 Wild R/T

      1 Wild R/T FBBO Gold Member FBBO Gold Member

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      Booster can absolutely over assist...
       
    • slepr1

      slepr1 Well-Known Member

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      How about the booster rod? May need adjustment.


       
    • 1 Wild R/T

      1 Wild R/T FBBO Gold Member FBBO Gold Member

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      He already covered that in the opening post..
       
    • shovelluv

      shovelluv Well-Known Member

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      Can the booster over assist be adjusted?
       
    • slepr1

      slepr1 Well-Known Member

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      My mistake. I need to slow down when I read.
       
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      1 Wild R/T FBBO Gold Member FBBO Gold Member

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      It's not an adjustment, it's a bad seal on an internal valve that controls the vacuum being applied to the diaphragm.. It's supposed to have a linear feel to the assist & instead it's strictly on or off...

      Sent it to Booster Dewey...

      Booster Dewey
      503-238-8882
      power brake booster exchange, inc.
      4533 SE Divison St. - Portland, OR 97206
       
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      • 66Satellite47

        66Satellite47 FBBO Gold Member FBBO Gold Member

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        Possibly dumb question, was the car originally a drum/drum car or a disc/drum car if the previous MC was disc/drum? Proportioning valve?
         
      • shovelluv

        shovelluv Well-Known Member

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        Fender tag shows power brake code B51 power brakes. not disc
         
      • 66Satellite47

        66Satellite47 FBBO Gold Member FBBO Gold Member

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        If the shoes are clean, check that proportioning valve, especially if you changed the MC.
         
      • steve340

        steve340 Well-Known Member

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        0.035" is too much. I was taught to set them up with about 0.003" clearance on the booster rod. You can go to zero provided the master cylinder piston comes all the way back past the compensating port. My current car is set at zero and has no issues.
        When you put your foot on the brake the booster is not regulated at all it just starts moving forward. With to much clearance it slams in to the master cylinder piston and you get a vicious application.
         
      • shovelluv

        shovelluv Well-Known Member

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        It was at .010. I moved it to .035 no difference
         
      • steve340

        steve340 Well-Known Member

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        I would close down the clearance again. No difference generally means you are on the wrong track.
        I would go back and test to see what happens and move on to the next thing. Not to too many changes at once.
        Are your brake adjusters in the drums up fairly close? The heavy spring goes on the front shoe they may be the wrong way around?
        The same thing can happen if the shoes move out rapidly a long way.
        In old boosters there was what was called a reaction pad. It was a hard rubber washer between the daiphram and the pushrod. On early cars it could drop out and this made the brake application pretty vicious. On certain cars when you removed the master cylinder you had to check it did not drop into the booster casing. Perhaps yours is missing - worth investigation.?
         
      • BB BELLA

        BB BELLA Well-Known Member

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        I remember driving a new 67 monaco around a parking lot when I was kid and the brake pedal was very light with very little travel,everyone who drove the car took some time getting use to it.
         
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        • shovelluv

          shovelluv Well-Known Member

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          UPDATE... I pulled and sent the booster to harmon classic brakes. I talked to the gentleman on the phone and he explained to me if the master is leaking over the years and brake fluid gets into the booster it will over assist. I had him do a full rebuild on it, I reinstalled it yesterday and foot pressure on the brake pedal is normal now. Thanks for the help everyone! https://harmonclassicbrakes.com/
           
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          • Cranky

            Cranky Banned Staff Member FBBO Gold Member

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            Cool beans!! ChryCo usually did have brakes that were more sensitive vs the other makes and same way on the power steering. If you were a 'hot dogger' like I was, all of that was a plus for doing spins, donuts, fishtailing and the like :D
             
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