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4 Wheel disc

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  • 4 Wheel disc

    I just installed disc brakes on the rear of my "79" scout ll (scout front discs and calipers and modified front brackets) and am having a problem with the rear end locking up under heavy braking. I have the stock proportioning valve and have added an adjustable prop. valve (used from a friend) for the rear and it
    hasn't seemed to help. Do I have a bad adj. prop. valve or what? any input would help. Thanks

  • #2
    I may be wrong here but it is my understanding that the adjustible proportioning valve that you want to use for this type of deal replaces the original one. You only have so much presure going to the rear (orig. drum) and it is less than what goes to the front disk. So when you are putting an aditional proportioning valve after the orig. one you are not going to be able to get the pressure needed to operate the calipers. Am I making sense here? Let me know if i am not being clear enough.



    • #3

      The problem you are describing is the opposite of what my problem is. when I brake hard, my rears lock up. Too much pressure ! also the stock proportioning valve is a "combination valve" which also serves to activate the brake warning light on the dash and can't be replaced by just the adjustable prop. valve.


      • #4

        Oh, sorry must have miss read. So you have the adjustible valve in line after the original one and all of the adjustments cause the rear to lock up under heavy braking. Now are they locking up under moderate braking or under very heavy braking. I know that when it comes to fast stopping you want the rear to lock before the front because it keeps you going strait.


        • #5

          Any time I need to stop in a shorter than average distance I need to be careful not to lock up my rears.
          by the way they are 35-12.50s that I am lockinc up with ease!!


          • #6
            Now I guess it's my turn to muddy the waters a little. A proportioning valve is generally used only on disc/drum systems because of the disparity between discs and drums. A drum/drum system uses a simple distribution block and equal size lines to all four brakes. I think a disc/disc system should also use equal size lines and no proportioning valve. There are distribution blocks available with the switch for the brake warning light.
            Jim Price

            "The difference between genius and stupidity is that genius has limits."
            --Albert Einstein

            78 Scout Traveler, 345, TF727
            76 Scout II, 345, TF727
            65 1200 Pickup, 304, T-19, 4x4
            48 KB-8-F, RD-450


            • #7
              So I should look for a distribution block (combo valve) from a 4 wheel disc vehicle ?


              • #8
                drum brakes require a residual pressure valve to hold slight pressure on the wheel cylinders. It is part of the maser cylinder, it is suppose to be easily removed. It can cause disk brakes to drag and wear out fast, also probably can make them come on sooner than the front knowing they're allready touching.
                The proportioning valve is tailored to the vehicle and brakes combined, installing one off another vehicle may be for the better or worse. The light is useless, by the time it comes on the pedal is at the floor and at that point you know there's problems allready. So I'd ditch the original proportioning valve, connect the lines for the front, install the adjustable valve in the rear line and adjust the valve till you're not locking up.