4 wheel disc brakes and a MANUAL master cylinder !

Brakes, Steering & Suspension

  1. Kern Dog

    Kern Dog FBBO Gold Member FBBO Gold Member

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    Hey guys and gals...
    My '70 Charger has 4 wheel disc brakes and and power assist.
    The front calipers have a 2.75" single piston, the rears have a 1.5" single piston. The Booster and master cylinder are stock for a 1975 Dart with a 15/16" master cylinder. Yeah, it is a disc/drum master cylinder but it works great. I do have a drum/drum distribution block, NOT a proportioning valve. This setup may sound odd, but the brakes are great.
    I want to convert to a manual master cylinder for simplicity, weight savings (15 lbs!) and to be able to use a threaded-adjustable pushrod to tailor the pedal height to my liking.
    In 2012, I wanted to see how the car would perform with manual brakes. I tried 4 different manual master cylinders with sizes between 15/16" and 1 1/8". Each one resulted in a firm pedal but terrible braking. The best of the 4 was just barely able to skid on a dirt surface. I found later that all along, I had a disc/drum proportioning valve in the system. The power booster must have masked the mismatch. I gave up on the manual brake idea and put the power stuff back on.
    Now I am considering a "do-over" with the manual brake setup.
    I do not want to take a step backward in terms of braking feel or performance.
    I have dealt with Dr Diff before and have had good results.
    Does anyone here have a 4 wheel disc setup with a manual master cylinder in place?
    Anyone here have experience with this?

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  2. 65-440

    65-440 Well-Known Member

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    I'll be following this one. My buddy has a 4 wheel manual willwood setup on his car now and has been battling crappy brakes. Pedal was hard as a rock at 1st with poor stopping performance, discovered the difference in brake pedal leverage b/t power and manual brakes. Fixed that, now the pedal is much softer but still struggles to stop the car in a hurry. Honestly its scary to drive, pedal will slowly sink all the way down while the car creeps to a stop. Not even close to locking up. He does have a fair amount of steel braided brake line, he's in the process of making all new steel lines now so we'll see how that goes.
     
  3. chtampa

    chtampa FBBO Gold Member FBBO Gold Member

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    Did the same thing and I hated how hard that I had to push the pedal. I added the booster and was happy then.
     
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    • yella71

      yella71 Well-Known Member

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      do your self a favor and keep the power assist . the only thing manual you want is your trans
       
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      • Darter6

        Darter6 FBBO Gold Member FBBO Gold Member

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        My 65 has SSBC 4 wheel manual discs installed back in 2004. Using their master with a adjustable proportioning valve for the rear.
        When you drive late model stuff daily then jump in the old car you swear the thing will never stop.
        I too have been thinking of doing a change over to a power booster.
        Kern, my vote is to stay with the booster.
         
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        • steve340

          steve340 Well-Known Member

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          I would also say stick to the booster.
          Two things are relevant in my mind and on a standard vehicle unmodified there is not much you can do to fix this. I am not a brake expert and I will probably get/explain something wrong.
          Drum brakes have what is termed mechanical advantage which is essentially the shoes "wind" in to the brake drum adding to the force the driver is applying to the brakes. The result is stand on the pedal hard enough and if you have big diameter/wide drums they can be effective until they heat up and you get brake fade. Disc brakes have no mechanical advantage therefore they require more line pressure to work properly.
          The second is pedal ratio which on most standard cars is low. The way I think about this is by thinking long wrecking bar versus a short bar.
          The booster gives you more line pressure with essentially less leverage/force applied to the pedal. The booster adds to the pressure supplied by the driver.
          My race car has 5:1 pedal ratio and I can generate about 800 to 1000 psi in the front system if I really stand on them. It has good brakes because it has massive rotors and callipers with large pad surface area. The car only weigh's 1050 kg which is a big factor.
          A street car would probably be more than twice that weight. The more the car weighs the more "energy/effort" must be applied to stop it.
           
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          • 7t500

            7t500 Well-Known Member

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            I have stock style front disc's, rear SSBC disc's, a Dr Diff manual 15/16 aluminum master, stock drum/drum distribution block, and adjustable rear proportioning valve. All parts including lines/hoses were installed new in 2017 when I got it on the road. She stops extremely well and while it does require some pedal effort, I don’t find it excessive at all. I can easily lock up all 4 at the same time and wouldn't hesitate to do this same setup again. Hell, I even got the parking brakes to work.
             
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            • Kern Dog

              Kern Dog FBBO Gold Member FBBO Gold Member

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              See, the inclusion of the drum/drum distribution block and an aftermarket proportioning valve confuses me a bit.
              My understanding is that the reason that 4 wheel drum cars didn't need a proportioning valve is because the bore sizes of the wheel cylinders were properly chosen to achieve the right front/rear bias. I've read about using Dakota or Minivan rear wheel cylinders to reduce the risk of lockup.
              In the case of 4 wheel discs, No car that I know of came with them AND a manual master cylinder. The buying public embraced soft pedal effort many years ago and the brake booster achieved that.

              My efforts from 2012 are clouded by the use of a factory NON adjustable proportioning valve. I suspect that the pressure to the rear was limited by that valve, adding to the poor performance.
              I am tempted to give this another effort. I have a couple of aluminum master cylinders here, I just need the 4 to 2 bolt adapter. I can make the brake lines myself.
              If I encounter the same miserable results, I'll be honest about it. The appeal of this is a cleaner underhood appearance, less weight, a system not dependent on engine vacuum and more room to work on the engine...Spark plugs, valve covers, header gaskets, stuff like that.
               
            • Kern Dog

              Kern Dog FBBO Gold Member FBBO Gold Member

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              I just read this:
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              The reason aftermarket "proportioning" valves (that are nothing more than pressure regulators) are inappropriate is that they do not perform the multitude of functions a factory proportioning valve performs. There are several functions the factory prop valve performs that are unique to a disc/drum braking system and very necessary for properly functioning brakes.

              One is holdoff. Holdoff prevents brake pressure from being applied to the front brakes till a setpoint pressure level is reached in the rear brake lines. This setpoint is fairly low and the function of the holdoff feature is to allow the shoes to overcome the return springs and just touch the drums when the setpoint is reached. Running larger brakes will change the setpoint value. This allows the shoes and pads to apply simultaneously and prevent accelerated pad wear. An all drum car will not need this feature as all the brakes have the same "lag" to overcome and if the shoes are adjusted properly will all contact the drum simultaneously. An all disc car does not have this lag as the pads "drag" all the time and the application of braking force is "instantaneous" at all four wheels.

              Next is true proportioning. Proportioning requires a reference to front brake pressure to adjust rear brake pressure in PROPORTION to front brake pressure. Aftermarket "proportioning" valves do not do this. Why do this? As you know the front brakes to a majority of the braking. On an all drum or an all disc car you can build this in by changing the drum diameter, shoe width, wheel cylinder diameter, caliper bore diameter or pad size. Due to the characteristic differences between a disc brake setup and a self energizing drum brake setup it is simpler to use a proportioning valve to help the marriage of these two systems in the same vehicle. Older, all drum cars used a single outlet master cylinder to supply pressure to all four corners. The needed difference in braking abilities were accounted for by the physical sizes difference between the front and rear brakes. Either larger diameter or larger width front drums were used to balance the braking needs of the vehicle. When discs were added to the front then your pressure requirements changed and the prop valve was invented to accomodate this.

              Next is hinge point. This is a rear brake pressure setpoint as well. Once this setpoint is reached the factory proportining valve slows down the RATE OF increase on the rear line pressure. Due to the nature of SELF ENERGIZING drum brakes there comes a point in brake application that the drums will out brake the discs and lock up. The hinge point backs off the rate of increase so that the rears do not lock up before the fronts. For an all drum car hold off is not need as all four sets of shoes will dig in the same amount and the physical size of the front brakes compared to the rear brakes will account for the additional needed braking capacity in the front, this physical differences is what will cause the fronts to lock first in an all drum car. An all disc cars does not concern itself with this as the brakes are not self energizing.

              Self energising drum brakes tend to dig in harder for a given input pressure than non-self energising brakes. It's due to the physical design of the system that allows the shoes to partially rotate into the drum and lever themselves in tighter. This requires less brake pressure for a given stopping force than non-self energised drums.

              Then there is the brake warning circuit mentioned, it tells you if you lost one side or the other. To reset this I think you need to fix the cause of the pressure loss, bleed and it should auto reset once fixed, if it isn't stuck. You can take the valve apart to clean them. Becareful as it uses O rings to seal things up and parts need to go back together as it was taken apart. The safety circuit also closes of the leaky half of the system to maintain pedal height and retain some stopping ability.

              The only appropriate use for these aftermarket "proportioning" valves like the MP or Wilwood ones are in conjunction with a factory valve, not in place of one. The prop vlave setup in a factory application will vary depending on a number of factors and if you look you will see there are several part numbers for a given vehicle that accounts for the various braking options. My 87 Diplomat uses one of two prop valves, The difference is the std duty uses a 10" rear drum, the HD uses an 11" rear drum. Same exact front brakes.

              This is applicable to "modern" drum brake cars, modern meaning they have self energizing drum brakes. Older MoPars, like my 38 Plymouth do not have self energising drum brakes and as a lot slower to stop.

              The above assumes that the system is properly designed from the factory. This means that the front brakes, be it all drum or all disc, have the additional capacity they need to work. A bodies had a tendancy to lock up the rears on disc applications because the rear wheel cylinders were too large in diameter. Why? I think it's because when they designed the new disc system they didn't take the rears into much consideration and reused the existing B/E body 10" rear drum setup for the 73/up A's. This was too aggressive for the lighter weight of the A body and didn't take into account the different weight distributions of an A vs. the B/E models. Not to mention the different wheelbases changed the dynamic loading of the car, in effect the A body unloaded the rear brakes faster than the B/E models in a hard stop.

              A properly setup all disc or all drum brake system will not lock the rears up first. If it does you need to back off the bore diameter of the rear brakes, easiest to do on an all drum setup. Or change the diameter of the rotor/drum (decrease rear or increase front) or the pad/shoe compound (more aggressive up front, less aggressive in back). Then you can use an adjustable valve like the Wilwood to fine tune the system.

              On a dual reservior system the safety circuit does more than light up a warning lamp if you lose pressure in one half the system. The shuttle valve that actuates this light also closes off the port on the half that lost pressure. This keeps the pedal off the floor and allows for a reasonable pedal height so the remaining half can still function. There is way more to a factory prop valve than an aftermarket unit.
              *************************************************************************************************************

              This article touched on a couple of points that I have heard of before.
              The "Hold-off" reference is one. If the disc/drum proportioning valve I had in place did this, it meant that at low pedal movement, the front brakes were not getting immediate pressure since the rears were given some to "take up the slack" and overcome the brake springs. Secondly.....Then with more pedal pressure, the rears were shortchanged because the valve was intended to reduce rear line pressure to avoid lockup.
               
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              • steve340

                steve340 Well-Known Member

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                I know on the race car the brake bias is "roughly done" with the master cylinder and brake calliper piston size. This takes in to account the sizes of both front/rear.
                The final "tuning" is done with the balance bar on the pedal box.
                Some race cars also have a pressure reducing valve in the rear line but they may have drum rears which would need reduced pressure - not sure about this as I really have not looked that hard.
                Mine does not have a pressure reducing valve and many cars do not seem to have them.
                From your first post your system seems to do this also eg. have smaller weaker rears.
                You probably do not need a rear reducing valve.???? Why reduce the pressure to weaker brakes - not required.
                Also as you have four wheel discs no hold off valve would be needed.
                All you need and I have done this when adapting car in the past - is keep the distribution block/warning light and get rid of the rest.
                I still think you should keep your booster.
                 
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                • 71charger_fan

                  71charger_fan Well-Known Member

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                  I have Dr. Diff's rear disk brakes, the manual MC he recommended, and a distribution valve from a '73 Charger. Fronts are stock '77 Volare. I love the combination.
                   
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                  • chtampa

                    chtampa FBBO Gold Member FBBO Gold Member

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                    I gutted my Valve and turned it into a Distribution block.
                     
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                    • 7t500

                      7t500 Well-Known Member

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                      From what I remember doing my research was that no hold-off valve was needed and that most likely the adjustable rear proportioning valve was unnecessary but nice to have if fine tuning was required to help balance front/rear bias.
                       
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                      • andyf

                        andyf Well-Known Member

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                        I have manual 4 wheel disc brakes on both my Coronet and my Duster and they both work great. But, it isn't a combination that just falls together. You have to build the system so it has the correct balance front to rear. Typically the front needs about twice the braking force of the rear but the exact split depends on the weight distribution and how high the CG of the car is.

                        You can balance the front and rear brake force by changing the size of the rotors and/or the size of the caliper pistons. You can also tune the system by changing the pad friction.
                         
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