Re: Blleding Brakes 68 Corvette - NCRS Discussion Boards

Re: Blleding Brakes 68 Corvette

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

    Re: Blleding Brakes 68 Corvette

    Ronnie,
    I did exactly as you just described earlier this weekend and it worked fine on the bench and moved fluid.
    Does not however work in the car which is why I am so puzzled. Even with a vacuum there is no movement of fluid through the lines.
    Kurt
  • Mike McKown

    #2
    The junction block below the master cylinder

    has a piston inside that will move fore/aft. It will shift when you get a pressure differential between the front and rear wheels. If it shifts enough to turn the brake light on, on the dash (that's the purpose of the switch), it has one end or the other of the system blocked and will not allow fluid to flow. Loosen your lines from the master cylinder to the junction block at the block and see if you get pressure there when you pump the pedal. If you have pressure going in, check for pressure on the outlet side on the block.

    Mike

    Comment

    • Ronnie Robertson # 36786

      #3
      Re: The junction block below the master cylinder

      I agree with Mike. Start disconnecting lines, starting at the junction block, left front, etc. Or you could disconnect the MC and blow air into the bleeders on each caliber to check for free flow.

      Comment

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

        #4
        Re: The junction block below the master cylinder

        Mike-----

        I could be wrong about this, but, as I recall, the piston in the brake distribution block and pressure differential warning switch doesn't move far enough in either direction to block the "flow" of brake fluid. As a practical matter, if it did and one experienced a partial brake system failure in one of the two circuits, the piston would "finish off" that circuit resulting in an assured complete failure of that circuit.
        In Appreciation of John Hinckley

        Comment

        • Mike McKown

          #5
          Re: The junction block below the master cylinder

          I could be wrong also but I believe that is design intent. I based my suggestion on three examples:

          '77 Corvette. While bleeding the brakes on the rear with pedal pressure, the brake light popped on on the dash and fluid ceased to flow from the rear bleeder until I went to the front and opened a bleeder. The pressure pushed the piston back and lost contact with the switch. I had to do this several times to get the brakes bled.

          '79 Chevy 4X4 truck. Bought it new, never did have much back brakes. After two pad changes on the front, I pulled the rear drums and the linings were almost like new. I took the junction block apart and removed the piston and put it back together. Truck had brakes like it never had before.

          Brand X In the late '60's and early '70's when these blocks were first introduced, there was a nail head sticking out the front of the block that had to be pulled forward to retract the piston to allow fluid to flow to all wheels when doing a manual pressure fill of the system as opposed to evacuation/fill process.

          None of this proves anything about a '68 Corvette except it just reinforces what some engineer told me long ago. I dunno'

          In any case, loosening the lines from the junction block outward in the system will tell you where the fault lies. That is, assuming you have pressure at the lines to master cylinder.

          Mike

          Comment

          • Mike H.
            Expired
            • April 1, 2002
            • 57

            #6
            Re: The junction block below the master cylinder

            Joe,
            You are correct. I have recently had a couple of them apart, one dated in '71 and one dated in '72. The spool that completes the circuit, for the brake light, infact has springs on the outboard ends to recenter the spool when the brake pedal is released.

            Comment

            • Mike McKown

              #7
              Re: The junction block below the master cylinder

              Mike:

              I noted your answer about the springs which are used to center the plunger. The problem with them is they sometimes cannot overcome internal friction on the piston whether due to machining problems or gum or both. That's what was wrong with my truck that I used in my example. The piston was stuck and no fluid to the back brakes. MANY, MANY 70's Chevy truck owners had the same problem.

              Comment

              • Mike H.
                Expired
                • April 1, 2002
                • 57

                #8
                Re: The junction block below the master cylinder

                Mike,
                Sometime after '72, I am not sure of the year (Joe Lucia sure does), the proportioning valve was introduced. Then to bleed the brakes one had to center the valve, if it would not self center. The block we are talking about, unless the spring itself would become completely clogged, could not disrupt fluid flow as the spool, even with the spring fully compressed, will not reach the line ports. I examined this very closely, trying to determine why, after engine reaching operating temperature, the rear brakes started to apply themselves. That sure seemed like the only thing that would block fluid return to the master cyl. due to heat expansion. It turned out to be the .023 bleed hole in the master cyl. that was plugged (brakes worked fine until engine compartment heated up). I can see no way that the distribution block / warning switch, of this design, can disable one half of the system.

                Comment

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

                  #9
                  Re: The junction block below the master cylinder

                  Mike----

                  1978 was the year that the first COMBINATION VALVE was introduced to Corvettes. This valve incorportated brake fluid distribution, pressure differential warning, and a FIXED RATE proportioning function. The same valve was used through 1982. The proportioning function became necessary since, by 1978, Corvettes had become so heavy that the original front/rear brake proportioning, accomplished by the different size front and rear pistons, was in need of "modification". I'm sure that it was easier to add the proportioning function to the valve than it would have been to change the calipers and pistons.

                  The only other 53-82 Corvettes that ever had a brake proportioning valve were 1965-68 with J-56 HD brakes. This was also the only adjustable proportioning valve ever used on a Corvette.
                  In Appreciation of John Hinckley

                  Comment

                  • Mike McKown

                    #10
                    I Give, for now *NM*

                    Comment

                    • Michael W.
                      Expired
                      • April 1, 1997
                      • 4290

                      #11
                      No need to give-

                      my own experience, plus the cut away drawings I've seen indicates that there is indeed a tertiary, albeit seemingly superfluous function to the distribution block.

                      It appears that it is entirely possible, at least on some iterations of valves, that the flow to the 'leaking' side of the system becomes sealed off by movement of the piston within the valve housing. The movement is initiated by the pressure and flow of fluid from the 'good' side of the system. This is much like hydraulic fuses common within the aircraft industry.

                      I see no benefit to using a switch with this feature on a Corvette, and it may just be the root cause of Kurt's dilemma. Was there not some procedure on the board recently describing how to re-center the piston? Seems to me someone mentioned pulling the differential switch and in stalling a temporary rigid tool.

                      Comment

                      • Mike H.
                        Expired
                        • April 1, 2002
                        • 57

                        #12
                        Re: No need to give-

                        It's possible some iterations of the valve did work as you discribe, just not the iteration used on '69 to '72 Vettes (Passenger cars and pickups, due to thier weight, may have been different). I do not know if the '68 is the same or not. I would doubt, however, that it is significantly different.

                        Yes, it was necessary to pull the switch and install a temporary tool, however this was after the valve became a proportioning valve, as opposed to the one we are discussing here.

                        Comment

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