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Brake problems? don't overlook the proportioning valve

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  • Brake problems? don't overlook the proportioning valve

    A friend of mine who's just paid a pretty penny to have his corvette restored called me out of desperation (you know how chevy guys hate to ask ford guys for help) because his newly restored vette had been poured over by several mechanics at the restoration shop for a day and a half, and still the brake pedal was soft.

    They'd replaced the following:

    Master cylinder (twice after suspecting a bad rebuild on the first)

    LF caliper

    Both front flex hoses from caliper to hard line

    They'd bled the system multiple times using gravity, vacuum, and pressure.

    I asked if they'd tried to isolate the problem to one wheel by plugging master cylinder or proportioning valve ports one at a time.
    Yes - at least at the master - the problem appeared to be with the front, and likely the left front. (they used smooth jaw vice grips to pinch the rubber hoses to each caliper)

    I asked if the red brake light was illuminated on the dash.
    No - the light never came on.

    At this point by a matter of elimination - the proportioning valve was about the only thing left.

    The proportioning valve turned out to be the problem.

    I would have leaned toward a new aftermarket adjustable piece - but being a "correct" restoration - they found a good used one in the shop - cleaned it up - problem solved.

    My question is - what goes bad inside a proportioning valve? Is it something that can be "rebuilt" It's just a hydraulic piston with a couple springs and o-rings .... right?
    There's always something new to learn.

  • #2

    Re: Brake problems? don't overlook the proportioning valve

    I've wanted to rebuild one but no one knows the o-ring sizes or styles. I've had the o-ring go bad in one and blow out the one wire sensor. I say blow out, but what it was really doing is leaking like a bad oil pressure sensor every time I hit the pedal. They can get pretty rusty inside too. You've seen wheel cylinders with crud in them, the propotioning valve is the same way. I've disassembled two and didn't like what I saw inside either. The bores are too small to hone with any tool I know of. Maybe some scotchbrite pad material wrapped around a drill bit?
    Escaped on a technicality.


    • #3

      Re: Brake problems? don't overlook the proportioning valve

      In my opinion, at least the older ones did more fail-safe functioning than true proportioning.

      The proportioning was handled by making the rear line-size different that the fronts.

      The valve really only functioned if there was a loss in pressure on one side. The valve would then close off the side with the leak, keeping functionality in the other side of the system.

      That's my view on it. And I've had them go bad, the valve goes to one side and will not come back. There is a tool to release the thingy on the front, to manually recenter it, but the thingy on front is usually rusted too far for the tool to fit.
      Either the bore is so bad it sticks, or the springs rust away/go very weak. Either way its replacement time.

      I like a gutted valve with an aftermarket adjuster on the rear line. Simple and effective.
      Of all the paths you take in life - make sure a few of them are dirt.


      • #4

        Re: Brake problems? don't overlook the proportioning valve

        Here's a cutaway view, I hear ya - they don't lend themselves well to rebuilding without special tools...

        This has me thinking - I should probably get a prop valve from a lincoln versailles or something like that with 4 wheel disc.... since I'm putting M2 brakes up front and crown vic discs on the 8.8.
        Last edited by milner351; June 21st, 2012, 10:15 AM.
        There's always something new to learn.


        • #5

          Re: Brake problems? don't overlook the proportioning valve

          proportion valve is indeed simple.

          going full restore is not all that smart prior to the 2000s in the brake department.

          most vehicles upgraded with common snense..but some stuff is hellbent on staying on the shelves.

          Try a modern self bleed master cylinder. Non self bleeding favors a side instead of realeasing. proportion right at the source.

          I had a non self bleed in a 1987. the same thing. front left and soft pedal. it was the master cylinder. An easy guess, as proportion valve has been universally mastered since the 60s. 1987 had no excuse at all.
          Master cylinders have not mastered all that long..and hit or miss right into the 80s to have some antique.
          Last edited by Barry Donovan; June 21st, 2012, 10:16 AM.
          Previously boxer3main
          "Everything is on earth practically. All the good stuff." - Scott Kelly


          • #6

            Re: Brake problems? don't overlook the proportioning valve

            This would probably work in most cases for 4 wheel disc.... I'll probably end up with this one unless I can find one in a junkyard that's not metric fittings.

            There's always something new to learn.


            • #7

              Re: Brake problems? don't overlook the proportioning valve

              What year Corvette??....... I know the early 70's models had brake bleeders on the top and the bottom of the calipers and you have to open "all" of them to bleed the brakes properly........ We had a similar problem with a '71 vette that we did brakes on and that was the fix...........


              • #8

                Re: Brake problems? don't overlook the proportioning valve

                Good call TC - believe it or not - he's spent all this money on a '74 ... it's the car his wife was driving when they met, thankfully he ditched the smog 350 for a 350 horse version from jasper - but the rest of the thing is bone stock, just rebuilt.

                I asked about the bleeders - thinking they may have swapped sides with a caliper putting the bleeder screw at the bottom or something, but they had that all sorted out.

                The prop valve was the fix, but obeying murphies law, it was the last thing they tried.
                There's always something new to learn.


                • #9

                  Re: Brake problems? don't overlook the proportioning valve

                  Good call, John!

                  As far as honing really ANY bore - take a piece of welding wire (or a coat hanger in a pinch) and bend it around the center of a "flapper" made of roll abrasive - the kind with cloth backing. If you bend the wire back on itself and smack it with a hammer it'll be pretty small and you can make the abrasive flapper as big or small as you need it to fit in the bore. Hone away - I use brake fluid in a brake or juice clutch application. Granted it won't straighten the bore but that is usually not an issue. My usual experience is that once you get the rust out of there and a smooth surface (switch abrasive grit as needed).

                  As far as O-rings - take the dead ones to your local bearing supplier and they'll match them up. Tell them what fluid they need to contain and they'll come up with an appropriate material. Silicone O-rings will contain pretty much anything and are readily available.