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1968 Master Cylinder

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

    1968 Master Cylinder

    Where is the best place to buy a factory CORRECT master cylinder for a 1968 with power brakes.
    It will need to have all the correct codes and markings. I have one on my 68 coupe now but would like to replace it.
    Thank you,
    Kurt
  • Geoff C.
    Expired
    • May 31, 1979
    • 1613

    #2
    Re: new original or used original *NM*

    Comment

    • Joe R.
      Extremely Frequent Poster
      • March 1, 2002
      • 1356

      #3
      Re: 1968 Master Cylinder

      If the one on your car now is correct, have it resleeved and rebuilt by White Post Restorations in Virginia.

      Comment

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

        #4
        Re: 1968 Master Cylinder

        I already had it resleeved and that is why I want to replace it. It was resleeved and rebuilt by a reputable vendor who is known to this board and it does not work properly.
        Kurt

        Comment

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

          #5
          Re: new original or used original

          Geoff,
          Used original would be OK as long as it has the correct casting number, the 'PG' code, and a date code that is correct for my May 10, 1968 build date
          Kurt

          Comment

          • Geoff C.
            Expired
            • May 31, 1979
            • 1613

            #6

            Comment

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

              #7
              Re: new original or used original

              OK, here goes.
              Had it resleeved with stainless steel bore, and the same company rebuilt it at the same time. Total cost $97.50 and I supplied the master cylinder.
              When I drive my car, the pedal goes to the floor and I must downshift and use emergency brake to stop the car. After driving about 15-20 minutes, I finally get a PERFECT pedal and the car stops like it just came off the assembly line. If I drive the car again that day , no problems. If I let it sit and drive it a day or so later, same problem all over again until I drive 15-20 minutes. I have bled the brakes multiple times and no air. Also had the booster rebuilt by Booster Dewey so I don't think it's the booster
              My guess, anf that is all it is, is that the bore is not correct and that it takes many stops until somthing inside the bore swells up or expands to the point where it makes a good seal.Maybe the brake fluid heats up and swells the rubbers or the engine heat does it, I don't know. Either that or they assembled somthing wrong or used the wrong size internal rubbers. I am no expert, but once the brakes come back they are perfect with very little travel before they stop on a dime.
              That's why I want another master cylinder so I can start all over again.
              Kurt

              Comment

              • mike cobine

                #8
                Re: 1968 Master Cylinder

                What isn't working on it? They are usually known to work or not work, meaning they make and hold pressure and stop the car, or they won't hold pressure and the car doesn't stop.

                This is telling a dumb story on myself, but when I put mine back on the car after rebuilding, I cleaned the plunger rod and then put it back into the booster. I got the adjustment off, and the MC wouldn't work right. If you are having a problem, check that you have the rod adjusted right. A quarter turn can make a difference. It is a cheap fix.

                Comment

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

                  #9
                  Mike, see symptoms above *NM*

                  Comment

                  • Joe L.
                    Beyond Control Poster
                    • February 1, 1988
                    • 43203

                    #10
                    Re: new original or used original

                    Kurt-----

                    Setting up the requirement that you want an original master cylinder correct in every possible way will make your task quite difficult. If you would settle for a correct casting and broadcast code stamping, you're going to have a lot easier quest. Such a casting can be found on eBay quite regularly. Then, you'd have to have it sleeved and "start the process all over again". If you go this route, you'd be best advised to try a brass sleeved unit this time. ONE of the reasons that I generally don't recommend stainless steel sleeving is that, for master cylinders, the stainless steel sleeving process is more problematic. That's because the stainless steel sleeve has to be finish-machined in-situ, unlike caliper sleeves which are usually pre-machined prior to installation. Stainless is fairly difficult to machine, especially in this sort of configuration. So, problems can occur. Still, they're quite rare with major rebuilders and, quite frankly, I doubt that there really exists a problem with your master cylinder, as-is. However, it's a reasonable move to try another master cylinder at this point to see if that will resolve the problem.

                    Another option is a reproduction master cylinder. These are currently available for a fairly reasonable price for your application. They are properly broadcast code stamped and I believe that they can be date coded to order, too. I have looked at these master cylinders and they look quite good to me. Are they absolutely perfect and undetectable? I'd say it depends upon just how knowledgeable and fastidious the "detector" is.
                    In Appreciation of John Hinckley

                    Comment

                    • Geoff C.
                      Expired
                      • May 31, 1979
                      • 1613

                      #11
                      Re: new original or used original

                      Kurt,

                      I'm not a disc brake whiz. Seems I have heard similar symptoms as you described, and it was not the master at fault, but I'm not well versed.

                      Check the M/C reservoir and report if the fluid level is dropping?

                      I don't see how the bore would seal itself after driving. But what we really need is a professional Corvette wrench, and I'm not qualified.

                      Geoffrey

                      PS Inquiring minds want to know: what does the rebuilder tell you?

                      Comment

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

                        #12
                        Re: new original or used original

                        Geoffrey,
                        No change in fluid level. That was the first thing I checked. No response to e-mail I sent to rebuilder describing problem.
                        Kurt

                        Comment

                        • Don Izzo

                          #13
                          Re: 1968 Master Cylinder

                          Kurt, I guess I'll ask the question no one else has. Is this the only change to your braking system you made. Did you remove any of the rotors? Could this be a rotor run out problem as described in the service manual?

                          I had the same problem with my 68 after exchanging the calipers for stainless sleeved ones and rebuilding the e brakes. Three master cylinders later (encluding a new one from GM) before I got a spongy pedal . Still don't like it but the car stops. The point is every Corvette Afficianato I spoke to blamed it on rotor run out. And said I wouldn't be able to correct it until I sent the rear bearing assemblies out to be dialed in to correct rotor run out specs. I doubted that, and never did it, but never got the firm pedal back that I had before I drilled out the factory installed rotor rivets then reinstalled the rotors probably not indexing them in exactly the same locations as the factory had them.

                          By the way, I even had the system "vacume bleed" to ensure no air was trapped in the system.

                          One question though. I don't recall the M/C plunger in the booster being adjustable as someone here said. Can this be confirmed?

                          Don

                          Comment

                          • Joe R.
                            Extremely Frequent Poster
                            • March 1, 2002
                            • 1356

                            #14
                            Re: new original or used original

                            Hi Kurt:

                            In my view, your description of the problem certainly points to the MC. If the fluid level is not dropping, I think the only place in the system that can cause these symptoms is the MC.

                            I would expect that the company who rebuit the MC would back their product and provide telephone support and/or a second rebuild. You mention no response to an email, but did you call them?

                            I agree with the suggestion to make sure there is free play in the pushrod. I think there is a bleed hole that is only exposed when the piston is at the very outward (idle position) extreme of its travel. I've heard of other temperature-related things that can happen when this bleed hole is not exposed, although the symptoms differed slightly from your situation.

                            Comment

                            • Chuck S.
                              Expired
                              • April 1, 1992
                              • 4668

                              #15
                              Re: new original or used original

                              I also doubt it's the master cylinder, but even it is, the fact that it's been sleeved once shouldn't mean the sleeve can't be replaced, does it?

                              In your place, I would first eliminate the master cylinder as the problem source by removing and capping the front and rear brake lines, and then plumbing up some short dead-ended tubes equipped so you can bleed the air out of them. Once you have your "brake system" bled, pump the brake pedal (stationary in the garage, of course) and see if your pedal firmness still reacts the same...if your pedal stays firm, it's not the MC; if it is still starts off spongy, then either the MC or the booster is faulty.

                              Maybe you can disconnect the vacuum to the booster and see if that affects pedal firmness...I think without vacuum you should have a firm hydraulic pedal, but it will be pretty low in the travel.

                              Comment

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