K-H brake swap done, but doesnt stop!

Brakes for your Classic Mopar

  1. mopower440

    mopower440 Well-Known Member

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    Finally got the K-H swap and front end rebuild done today. The ONLY thing i have not changed yet is to the new disc/drum master cylinder as i still need to buy one. I couldnt resist taking it for a spin with the old drum MC. Well, it doesnt stop worth a darn! I have to literally stand on the pedal and it just comes to a slooow stop! I couldnt even lock it up in my gravel driveway doing 30 mph! I know it doesnt have the correct MC, but surely it should still have more stopping power even with that MC on it..? Any ideas?
     
  2. BigBlockMopar

    BigBlockMopar BigBlockMember

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    The pedal is firm?
    Not bottoming out?

    I think you might be underestimating the function of a correctly sized MC.
     
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    • Garrett Ellison

      Garrett Ellison Amateur driver on public roadway, do not imitate.. FABO Gold Member

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      Don't Drive It! The master cylinder doesn't have enough fluid volume to operate the calipers, all the fluid is going into them and not doing anything, well not doing anything significant anyway.. Well, I guess you might could use the emergency brake in conjunction and do a little something resembling stopping but is it really worth the risk of wrecking your beloved project for? The proper master cylinder make all the difference in the world. Care to tell us about the rest of the car?
       
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      • moparmat2000

        moparmat2000 FABO Gold Member FABO Gold Member

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        This!!!!
         
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        • 67Dart273

          67Dart273 FABO Gold Member FABO Gold Member

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          Nonsense

          Is the pedal "hard?" How far above the floor is the pedal with lots of foot pressure?

          OP you haven't told us a bloody thing here. What year is the car? What brakes did it COME with? Power? Manual? Did you install a metering/ proportioning valve? Not enough info............

          Also what condition are the rotors? New? What kind of pads did you use? And some pads need a bit of break in, a few miles............
           
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          • ir3333

            ir3333 Well-Known Member

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            273 is correct the volume in a drum master cylinder will work fine for discs
            2 things...If your brakes and lines are new or in good repair you either have a master cylinder piston that is too big or you haven't bedded your pads.Hard stop 3 times from 60 mph to 30 mph and then when cool haul it down from 30 mph to a stop a few times and you should be good.
            Manual discs need a master cylinder with a piston 15/16" or less.
             
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            • moparmat2000

              moparmat2000 FABO Gold Member FABO Gold Member

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              He said he cant get them to lock up on a gravel driveway at about 30 mph standing on the pedal, are you trying to kill the man ?!? Obviously there is something wrong. Change to the correct split circuit disc/drum master, and try it again.
               
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              • mopower440

                mopower440 Well-Known Member

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                Its a 1972 dart swinger, originally a 318 car,now a 440. It had manul 10" drums all the way around. I bought a newly rebuilt Kelsey hays setup years ago and finally got it installed. I replaced the old drum distribution block with a 1974 dart disc/drum proportioning valve, like doctor diff sells for disk swaps..The disk setup has rebuilt calipers and new pads and rotors..They have been sitting in my house and then my shop for the past 15 years since i bought them..They bled just fine and the pedal is very high and very firm! I have to put all of my 210 pound rear into the pedal to even get it to slow down..
                 
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                • moparmat2000

                  moparmat2000 FABO Gold Member FABO Gold Member

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                  I am thinking you would have been better off doing the BBP cordoba big brake conversion. More wheel choices, bigger wheel studs, easier to find replacement parts including rotors.
                   
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                  • billccm

                    billccm Well-Known Member

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                    Well back in the day when I was much younger and a bit more stupid, and no disc o tech brake swap articles we used to swap front disc from various cars onto our A and B body rides (think 1980s). No master cylinder change out, or proportioning block, etc. The car's braking was improved. Was it optimized? NO! Was it improved? YES! Not sure what the OP cause is here, but the cars would stop pretty good with just a spindle and caliper installation.
                     
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                    • Demonic

                      Demonic Well-Known Member

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                      So, they went to the 73 and up design because of sticking pistons in the KH calipers. I drove my Demon with brand new KH calipers back in the late 80's, and in just two years I had one stuck piston per side. Get a friend to SLOWLY push the pedal with a pad removed so you can watch the travel. Must be gentle to prevent the piston from popping out.
                       
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                      • ir3333

                        ir3333 Well-Known Member

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                        ..if the brakes and lines are new or in good repair means there is nothing wrong...changing to the correct master cylinder shouldn't kill him.He probably shouldn't try bedding the pads on a 12 lane freeway at rush hour though.
                        Good point.
                         
                        Last edited: Apr 21, 2019
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                        • Garrett Ellison

                          Garrett Ellison Amateur driver on public roadway, do not imitate.. FABO Gold Member

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                          Agreed, the fluid volume should be adequate for the early a disc brakes. However, if the rear brakes aren't locking up on gravel while standing on it, I would take a second look at the proportioning valve and rear brake adjustment, etc. Definitely check the caliper pistons, I sometimes drive a Tacoma on occasion with the infamous four piston calipers. When those things get stuck, stopping a sled is easier...
                           
                        • 67Dart273

                          67Dart273 FABO Gold Member FABO Gold Member

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                          Sounds to me like this is a simple matter of cylinder bore size. As others have said, I bet you'll need to add a prop valve.

                          To give you an example (I don't know what my car "is" for master size) I'm using 73/ 74 Duster/ Demon in front on my 67, and a disk brake factory 9" Ford out of a Versailles, so 4 wheel disk brakes. I "tried" t he factory master and it works just great. But we are talking 2 more disks and I don't know how the bore size compares to the newer cars.
                           
                        • moparmat2000

                          moparmat2000 FABO Gold Member FABO Gold Member

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                          Your right changing to a correct master cylinder shouldent kill him. However your initial post did not recommend that, only to bed them in at higher speeds which he currently cant do since the car wont stop very well the way it currently is.

                          You are correct with the basic fluid dynamics. If the piston in the master it too big, you will get a hard pedal and decreased grip from the calipers. Too small of a master cylinder bore will also give you decreased grip from the calipers with a soft pedal. The master needs to be sized just right for the amount of pedal force required and caliper grip required for the particular application.
                           
                          Last edited: Apr 22, 2019
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                          • mopower440

                            mopower440 Well-Known Member

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                            The more i think about how its acting, the more i think the caliper pistons are stuck from sitting so long..They were freshly rebuilt when i got them but that was 15 years ago..
                             
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                            • moparmat2000

                              moparmat2000 FABO Gold Member FABO Gold Member

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                              I guess its time for new seal kits and take em apart. I still say the cordoba setup is much better less moving parts, easier to buy replacement parts for. SBP 4 pot calipers. Id leave that stuff for the restoration guys.
                               
                            • ir3333

                              ir3333 Well-Known Member

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                              If the caliper pistons are stuck wouldn't it be difficult to bleeed?
                               
                            • jas0162

                              jas0162 Well-Known Member

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                              no
                               
                            • 67Dart273

                              67Dart273 FABO Gold Member FABO Gold Member

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                              You don't need to take them apart you can tell right on the car. Have someone apply the brakes while watching the calipers and turning the rotor. If that is not enough, pull one or the other caliper and retract the pistons. I just use scrap wood and a C clamp. These are dual piston? So you'll have to devise necessary adaptations. You should be able to easily retract the piston by using little force on the C clamp

                              Don't force them too far if you have the master filled up, or else remove some fluid first. Likewise, after retraction, either reinstall or put some wood between the caliper to mimic the rotor, and go apply the brake. Pedal should go down fairly easy to "take up the slack" and re-position the piston.
                               
                            • mopower440

                              mopower440 Well-Known Member

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                              Just went out and tried this and sure enough, the pistons are sticking..arent all 4 pistons supposed to come out when putting air pressure to the calipers?? on one of the calipers, only one side comes out and is hell to get to go back in and the other caliper they are sticky on some of the pistons..thinking of just rebuilding them as its a lot cheaper, just afraid of messing the seal up when installing the pistons..
                               
                            • Demonic

                              Demonic Well-Known Member

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                              You have to hold the first one and see if the others will move. But it doesn't sound good.
                               
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                              • Logan

                                Logan FABO Gold Member FABO Gold Member

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                                Did you install an adjustable push rod that has a fat heim joint that attaches to the brake pedal? I had the same problem as you and after a lot of screwing around found the heim joint was binding against the brake pedal and wouldn't allow full travel of the pedal. A little grinding on both the pedal and heim joint removed the interferance and the brakes work just fine now. My set up is manual with dual resevoir master with 15/16 bore. Not KH though, just 73-76 Mopar.
                                By the way, it doesn't seem unusual to me for just one piston to move when testing the caliper. It just happens to be the one that over came friction first and then just kept moving with reduced friction.
                                 
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                                • moparownr

                                  moparownr FABO Gold Member FABO Gold Member

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                                  I have had these calipers since 1980, (when I bought my Barracuda Convertible) they are prone to sticking, you bought these 15 years ago?? check the pistons, there were alot of them produced with a chrome plated piston. The chrome plating will rust off of these pistons and stick them in the holes, make sure if you have them to replace them with plain steel ones, they last longer and work smoother.... hope this is helpful.
                                   
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                                  • mopower440

                                    mopower440 Well-Known Member

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                                    well, got them rebuilt, but never again! I will use them for the time being but will be looking for some 1973-76 A-body calipers, rotors and spindle/hub so next time these calipers need rebuilding, i will swap it all out for the single piston style. I fought with thise damn dust boots till i was blue in the face, trying to get them to stay put while installing the pistons..what a pain in the rear! FINALLY got them done.
                                     
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