Brake Pedal STILL Sinks..

Brakes, Steering & Suspension

  1. Ron H

    Ron H FBBO Gold Member FBBO Gold Member

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    I posted earlier on some reasons why the brake pedal sinks; my '63 has a front brake disc kit in it with booster converting to power brakes. Had to re-do the brake pedal as it was real high cutting down the push rod and installing a better eyebolt. Pedal sinks when starting the car until pumping a few times but having to put foot in back of pedal to bring it up. The brakes though are not engaged when this happens. After some driving time same thing happens on occasion. Installed an auxiliary reservoir I got from Jegs. Same friggin problem. Even put in a return spring and same thing still happens. A heavier spring is likely needed; but chit shouldn't need one to begin with. Have ample vacuum and booster holds pressure hearing a quick whoosh when disconnecting the connection. If brakes would lock when this happens that's one deal; but loose the pedal almost to floor but can still engage brakes. Prop valve seems adjusted fine...nice braking action, no leaks anywhere. Any other thoughts on the fix? Thanks
     
  2. coronet68mx

    coronet68mx FBBO Gold Member FBBO Gold Member

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    It's a pain in the ass but if that was my car I re check the connection rod and see how far push down the brake cylinder , now if it pushes down enough even a new brake master cylinder can fail, the most common would be the o' ring around the cylinder inside the brake assembly, like any other cylinder needs a rings to hold the pressure, due time or wear this fails, if this part cant hold the pressure then the pedal goes to the floor and needs to pump the pedal several times in order to regain pressure, now the woosh is most likely the brake booster that can't hold vacuum.
     
  3. wagonman

    wagonman FBBO Gold Member FBBO Gold Member

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    Is there a pin between the master and booster? Some of the generic boosters based on a GM design have them, some are adjustable and some are a select fit. Make sure the master is for a power app and not a manual style. I think the power one will have a flat piston with a small indent to center the pin and the manual will be deep to hold the original rod with the small retaining bushing. Just spit balling here.
     
  4. snakeoil24

    snakeoil24 FBBO Gold Member FBBO Gold Member

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    One more thought here to check if the seals are leaking past the piston in master cylinder no matter what you do your never going to fix. plug the ports on brake lines and then press down holding with steady but light pressure see what happens.
     
  5. toolmanmike

    toolmanmike Moderator Staff Member FBBO Gold Member

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    If you can pump the pedal and it holds, there's air in the system. If it doesn't get solid and settles to the floor with steady pedal pressure it's in the master cylinder.
     
    Last edited: Aug 14, 2019
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    • Ron H

      Ron H FBBO Gold Member FBBO Gold Member

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      Buddy told me if I can't cause the brakes to skid on pavement on panic stopping this is a problem. I can't skid the brakes and just barely can on gravel. If I run it in reverse the rear brakes will do some skidding. Not sure wtf this means but then my track record figuring out the brake problem has been piss poor so far. On the post about my hearing a little whoosh when unplugging the booster line valve I thought this was a good sign; but guess it's a no?
       
    • RJRENTON

      RJRENTON FBBO Gold Member FBBO Gold Member

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      Response #5 is a good gauge of the problem.
      The above quote re skidding is sort of lame....too many variables....like: car's weight distribution - front to rear, what size is the gravel, moisture level, what is the tire pressure and tread pattern, how much brake pressure is applied (measured at both sections)....the comment is basically worthless. Perhaps your "buddy" can offer some additional information to substantiate his comment. Is the proportional valve is faulty or the rear wheel cylinders are not the correct bore size, or the master cylinder has the wrong size piston. Skidding on gravel means nothing. Just my opinion of course.
      Bob Renton
       
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      • Ron H

        Ron H FBBO Gold Member FBBO Gold Member

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        Yeah I thought so too - getting to grabbing at straws just in some wildly remote case it is actually non-BS. Well had bled the system twice and now looks like I need to gaze at the rod adj btw the booster and MC (did this months ago and can't even recollect what I did anymore, whatever it was it did jack anyway). I've got replies that the MC could be bad and gather there's no way to be certain on this unless something obvious and as well the booster could be chit too. It was an all 'new' not cheap kit...
         
      • RJRENTON

        RJRENTON FBBO Gold Member FBBO Gold Member

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        Ron,
        IMHO, you should document your attempts to fix a problem, in a logical order and their results. That way you won't be influenced by feeble comments or waisting your time on some things that mean nothing....afterall its your time and money and safety. Stopping is just as important as the other end of the spectrum.....going!
        Bob Renton
         
      • Ron H

        Ron H FBBO Gold Member FBBO Gold Member

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        I'm getting an education much as I've done car resto's over the decades and the brake hassle has been a head scratcher. Doing some long web surfing on this it's amazing how many 'solved' their problem adding a return spring and while I did this, the spring I used isn't strong enough to override the pedal sink. Except this doesn't set well...adding the spring is a band aid over some deeper problem. So next step is to experiment with a spacer between the booster and MC to see what this does. If this is fruitful it's a matter of correct adjustment of the rod. Or finding that the rod needs adjust in the other direction much as I thought I found the sweet spot with earlier adjusting on the pedal rod. If this doesn't pan out at least I gave this a go before getting more radical and expensive replacing the booster and/or MC...
         
      • Jeff Brundage

        Jeff Brundage FBBO Gold Member FBBO Gold Member

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        I chased a similar problem with a car without a booster. It ended up being the master. Even brand new stuff can be bad. Especially if its a cheepo offshore part.
         
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        • miller

          miller Well-Known Member

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          Too many parts of the puzzle. Though, since it's a main factor in the system, I'd sure consider the MC. Is it the right type...for a booster disc/drum? Even so, if it's right, is it together right, with the correct parts? All internal seals good? (Sounds like it's not up to snuff)

          I've been into several masters, so not that big a deal. If you've got a part # for it, you can probably find a break-down diagram for it. Just saying...you pull an assembly like that out of the box, doesn't mean the 'assembler' got it right.

          Good luck with it! Yep, need good stoppers!
           
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          • Ron H

            Ron H FBBO Gold Member FBBO Gold Member

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            Yeah - just trying to rule out any culprits that are more simple, though running low on those, before the major stuff. It was a pricy kit from SSBC and they were of little help to me diagnosing the problem on the phone. Well it's not their car so who cares I reckon. I'll have to do the check mentioned on the booster type as I open this up.
             
          • Ron H

            Ron H FBBO Gold Member FBBO Gold Member

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            The saga goes on. I did the three often recommended vacuum tests on my booster and all passed just fine. Trying to connect the dots on this, I would a lost the bet on this. When I separated the boost from the MC I pressed the pedal a dozen or more times and the pedal would spring back to the return position every time. This should tell me that the pedal no-return I have has nothing to do with the booster. And even if the MC piston might not return or air in the system (my next step is bench bleed) the pedal still should spring back? As mentioned, the pedal will sink down and not return; but does not engage the brakes. I can still apply the brakes fine with the pedal sunk. So then wtf could cause the pedal to do this? My thoughts as others have put me onto include low vacuum. I have a slightly mod cam and find my vacuum (installed a gauge) runs often around 15-16 while cruising so also reckon going from 3:23's to 3:55's didn't help the case as now higher RPM's = less vacuum. Ok - well I installed a vacuum reserve tank and this has done squat! Except others have said they did this and solved their low vac to booster problem. My setup from SSBC is a front disk retaining my drums in rear. A couple of calls to these folks didn't offer much help or sadly, much concern. So here I am after a year and a half without finding the culprit...any other tips are greatly appreciated!
             
          • 69L48Z27

            69L48Z27 FBBO Gold Member FBBO Gold Member

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            What diagnostics did you do on the MC. If the booster alone returns the pedal it sounds like the MC is passing fluid past the seals making it not effective stopping the car and then not returning because it has fluid behind the seals.

            I didn’t read the entire thread but I see you mention bench bleeding, leads me to believe you haven’t spent time troubleshooting the MC. Your vacuum level should be fine to operate a booster, low vacuum just doesn’t have as much assist.

            Remove the MC and bench bleed, make sure it returns. Place back on the car and close off or block the ports out. Stand on the pedal. If it’s rock hard and doesn’t fall then your MC sounds good. If it falls and doesn’t return then your MC is toast. Past that, you could get a pressure gauge for the bleeder ports and see what your getting at the WC or caliper.
             
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            • matthon

              matthon Well-Known Member

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              Make sure you have the correct type of hose for the vaccum.
              Power brake hose is specifically designed to not collapse inside.

              Any old hose will appear fine, and may work under certain conditions, but will ultimately cause problems.

              I've also heard people buy those power disc kits, immediately toss the booster from the kit, and source another one.
               
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              • steve from staten island

                steve from staten island FBBO Gold Member FBBO Gold Member

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                I have the SSBC front disc conversion kit and I agree there tech department leaves a lot to be desired. Dont know if this helps but here's my experience. Brakes would not lock up, car would not stop safely. I was afraid to drive over 25 mph. I had a single diaphragm power booster from Dewey. I changed to a dual diaphragm booster which helped a little. It was only when I checked the pressure at the calipers while someone held the pedal down did I find I did not have enough pressure to the calipers.
                I spoke to Cass at Doctor Differential and I bought a 15/16 bore master cylinder from him. This did the trick and now I had adequate pressure at the calipers. Brakes are much better.
                 
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                • steve from staten island

                  steve from staten island FBBO Gold Member FBBO Gold Member

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                  Summit sells a kit which has a gauge and various adapters one of which should screw into your bleeder hole. Have someone step down hard and hold the pedal and check the pressure....
                   
                • miller

                  miller Well-Known Member

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                  Tried to look up, what your MC looks like, on the SSBC site...no tango. Just what does the master look like?
                  To me, still sounds like your prob is in it. Even new, dang thing could have been put together wrong. It either works, or it don't!
                   
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                  • Ron H

                    Ron H FBBO Gold Member FBBO Gold Member

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                    Here are a few photos; has an adj booster rod and I measured bore dia just over an inch (have to check my notes). Delco MC..

                    Brake MC 1.jpg Brake Master Cylinder Bore.jpg Brake Booster Rod.jpg Brake MC.jpg
                     
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