68 Brake Master Cylinder Question - NCRS Discussion Boards

68 Brake Master Cylinder Question

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  • Kurt B.
    Very Frequent User
    • July 31, 1996
    • 971

    68 Brake Master Cylinder Question

    I had my factory correct part number master cylinder resleeved and rebuilt.
    (Power Brakes) I also had the brake booster rebuilt, yet I have the following symptom which I need help with.
    The car is only driven on sunny days and weekends and often sits for extended periods. When first leaving the driveway, the pedal goes to the floor before the car will stop and only after driving for several minutes(or miles) will the pedal gradually get stiffer and stiffer until finally it is perfect and the brakes stop the car perfectly every time. It remains perfect for as long and as often as I drive the car that day and then the problem manifests itself again the next week or two when I drive it again.
    I have bled all the brakes several times and after initial bleeding, no more air was expelled. Also, when this happens, repeated pumping of the brakes does not cure the problem so I do not think it is air in the system.
    Could it be that something inside the master cylinder needs to warm up and expand to work properly. I did not have this problem prior to having the master cylinder rebuilt and resleeved. I want to keep my present master cylinder as it has all the correct part numbers and markings for my stock L79 68.
    Thank you for any thoughts or suggestions,
    Kurt
  • Jim T.
    Expired
    • March 1, 1993
    • 5351

    #2
    Re: 68 Brake Master Cylinder Question

    Kurt I have lost brake pedal due to real calipers sucking airon my 70 due to excessive disc runout, but always had some pedal on the front braking system that would stop the car. Your symptom does not match that. I have had only one brake booster go bad and that was on my 85. The sympton was when applying the brakes while stopping the pedal would go down and I could hear the air leaking. The valve that is inserted into the power brake booster is a check valve to maintain pressure in the booster. It may have failed. After you shut off the car the power brake booster should have enough reserve to stop the car, at least my 70 does. You will never know how your master cylinder was assembled unless you remove it and inspect it. It is another thing to check, new seal kit is easy to install, have to get the seals installed correctly, they will go on 180 out just as easy as the right way. Since the booster retaining nuts are many turns to remove I would spend the afternoon checking out the master cylinder since it is the easiest to verify. I am no brake expert by any means, mayby someone else can nail your problem.

    Comment

    • Kurt B.
      Very Frequent User
      • July 31, 1996
      • 971

      #3
      Re: 68 Brake Master Cylinder Question

      Jim,
      If the seals in the master cylinder were installed incorrectly (180 degrees off) would the car still stop?
      How do I know what the correct orientation of the seals is?
      Thanks,
      Kurt

      Comment

      • Jim T.
        Expired
        • March 1, 1993
        • 5351

        #4
        Re: 68 Brake Master Cylinder Question

        Kurt I have rebuilt master cylinders but have never got the seals 180 out, so I have not experienced using a master cylinder with one seal 180 out. Just read the instructions on seal installation. The lip of seal has to be forced out and seal on the cylinder wall when the piston is pushed down. Another thing I have done in the past Kurt is cap your brake lines removed from the master cylinder. When reinstalling the lines, do not tighten, loose fit, and let gravity bleed fluid into the connection, then tighten, in most cases you will not have to bleed the brakes at the wheel cylinders. If you properly bench bleed the master cylinder before putting it back the car, you will save some time and work.

        Comment

        • Joe L.
          Beyond Control Poster
          • February 1, 1988
          • 43193

          #5
          Re: 68 Brake Master Cylinder Question

          Kurt-----

          The attached shows the correct orientation of the seals and other components of the master cylinder for a Delco-Moraine master cylinder.

          The brake booster may not be the same. This is a Delco Moraine master cylinder and booster. The 1968-76 Corvette master cylinder was a Bendix master cylinder and may vary from this configuration.




          Attached Files
          In Appreciation of John Hinckley

          Comment

          • Kurt B.
            Very Frequent User
            • July 31, 1996
            • 971

            #6
            Thank You Jim & Joe *NM*

            Comment

            • John H.
              Beyond Control Poster
              • December 1, 1997
              • 16513

              #7
              Re: 68 Brake Master Cylinder Question

              When a brake pedal actually "goes to the floor" and no leaks or air in the system are evident, fluid is bypassing the seals or the seals have failed; can't happen otherwise.

              Comment

              • Terry F.
                Expired
                • September 30, 1992
                • 2061

                #8
                Re: 68 Brake Master Cylinder Question

                Joe, I seem to recall that the corvette didn't use a re*** valve at each of the ports of the master cylinder. Do you recall anything like that? I remember my old AC Delco rebuild kits comming with a little screw to extract the valves if they were pressed in there?? Terry

                Comment

                • Joe L.
                  Beyond Control Poster
                  • February 1, 1988
                  • 43193

                  #9
                  Re: 68 Brake Master Cylinder Question

                  Terry-----

                  67+ Corvette master cylinders use no anti-return valves at the master cylinder ports.
                  In Appreciation of John Hinckley

                  Comment

                  • Terry F.
                    Expired
                    • September 30, 1992
                    • 2061

                    #10
                    Thought so. Thanks, Terry *NM*

                    Comment

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