Which master cylinder for 4 wheel disk brakes

Brakes for your Classic Mopar

  1. prorac1

    prorac1 Well-Known Member

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    We are switching our project 63 valiant over to 74 E body front disk brakes, and a Ford 8.8 explorer axle with disks as well. What master cylinder is everyone using with 4 wheel disk brakes? Any help would be appreciated. FWIW were keeping manual brakes. Thanks, Eric L
     
  2. gold66cuda

    gold66cuda Well-Known Member

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    I run a 15/16" on my 66 with four wheel disc, stops great, good pedal feel and I have run it hard. I have run the 1 1/16" but didn't like the feel. Ten people will give you 10 different opinions on this subject.
     
  3. prorac1

    prorac1 Well-Known Member

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    Thanks for the reply. I wasn't sure if people were running a factory replacement from another car, or just going off of bore size with a wildwood or mopar performance. Thanks, Eric L
     
  4. Locomotion

    Locomotion Well-Known Member

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    I've always heard that disc brake calipers have bigger volumes than drum brake wheel cylinders so they require bigger bore master cylinders (disc reservoir is bigger than drum reservoir), but I've never had that combo. I use a lightweight Cardone Aspen/Volare manual disc/drum M/C 10-1821 on my race car. A bigger manual disc/drum version for trucks is 10-1860. An adapter may be needed. But as noted, the "feel" boils down to personal preference. Bigger = firmer/smaller = softer pedal. An adjustable proportioning valve can also help. But there are less expensive options than the big name brands.
     
  5. 67Dart273

    67Dart273 FABO Gold Member FABO Gold Member

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    Here's what worked for me, I realized different than OP. I did this just to try it and get the car moving, and it turned out OK

    I'm running 73/ 74 front OEM brakes, and a Versailles disc brake rear axle. I simply ruptured the residual valves in the master and used the factory 67 Dart master. It works very well in my case.
     
  6. halifaxhops

    halifaxhops It's going to get stupid around here! FABO Gold Member

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    I have the Cherokee rear disks and use the jeep 15/16 master and a dr diff adapter plate with a adjustable prop valve, stops great.
     
  7. silverex

    silverex Well-Known Member

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    like 67dart273 residual valve punctured made my rear disc work better

    got 11,75 frt and 11 rear disc with stock pb master and proportioning valves , also put a ajustable rear valve that is wide open !
     
  8. nm9stheham

    nm9stheham Well-Known Member

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    Disc brakes require a bigger reservoir because as the pads wear, the caliper pistons move further out of the bores, and the extra fluid is needed to fill the extra space in the caliper's bore. This does not happen with wheel cylinders in drum brakes; the adjustments to compensate shoe wear are in the mechanical parts, not in the hydraulics.

    Bigger master cylinder diameters reduce the hydraulic mechanical advantage, and so you have to push harder on the pedal to get the same stopping power at the wheels (or brake torque). A master cylinder bore of 1-1/32" diameter requires about 18% higher pedal force than a 15/16" bore, and requires more pedal travel change to modulate the braking force, so that is why most folks like the smaller bore MC.

    Using an adapter plate has one disadvantage: It move the MC forward and the brake pedal will be closer to the floor, typically by more than an inch. So, I like the standard Mopar MC's that bolts to the firewall without an adapter for that reason.

    Some level of proportioning change/adjustment is almost guaranteed. The smaller caliper bore size of the 8.8" rear calipers does a lot of the proportioning. An adjustable proportioning valve in the rear line is a common way, but I very much prefer to adjust the pad materials front versus rear to achieve this. It results in much more consistent F-to-R proportioning. We are getting ready to do this on my son's '65 Cuda with new front pad materials, to try to eliminate the adjustable proportioning valve i the rear line, or at least almost adjust it all out.
     
  9. BillGrissom

    BillGrissom Well-Known Member

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    Not for me. I used a newer MC w/ adapter plate on my 1964 Valiant, and the pedal is in the same place (photo, your results may vary). The MC is 7/8"D bore, which is the smallest I have seen (i.e. easier pedal). MC is for a 95-99 Mopar "cloud car" w/ ABS (Breeze, Cirrus, Stratus), and probably many others would work. I super-glued the 65 boot to the new MC (2nd photo). The bracket is from an Intrepid, on which I mounted an adj. proportioning valve. Haven't driven the car like this yet.
     

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  10. Rapid Robert

    Rapid Robert Well-Known Member

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    the rear piston "thimble" recess in the MC at the bottom in your situation must be shallower by the same amt as the thickness of the adapter plate for the pushrod length to be dead on the same in both. In my exp like NM9 the recess was deeper & I needed a longer pushrod.
     
  11. goldduster318

    goldduster318 Overzealous Car Modifier

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    Cloud cars have 50/50 split master cylinders because they are diagonal split cars (FR/RL on one circuit and FL/RR on the other).

    I highly advise against using one of these. You'll want a biased master cylinder from a F/R split vehicle. This could be an F/J/M body, a Jeep Wrangler, Chrysler 300, etc. All a better choice.

    I have a 15/16 dakota master cylinder from Dr Diff
     
  12. prorac1

    prorac1 Well-Known Member

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    Thank you for the replies everyone. I have heard of using the Cherokee master cylinder before and was considering that option.

    What problems do you run into not running a prop valve? Do the backs grab before the fronts because of the smaller caliper bore?

    Thanks again for the replys. We appreciate the help. Thanks, Eric L
     
  13. prorac1

    prorac1 Well-Known Member

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    And what year jeep Cherokee master is being used? Eric L
     
  14. DoctorDiff

    DoctorDiff Well-Known Member

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    If the adapter plate is designed correctly, you will not need a longer push-rod when used with an aluminum Mopar 2 bolt master cylinder designed to accept a manual brake push-rod.
     
  15. Rapid Robert

    Rapid Robert Well-Known Member

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    Eric I ran 76 (2.75) A body front discs with aftermarket (ceramic?) pads and 10" BBP 7&1/4 drum brakes in the rear (2&1/4"?) on a 65 dart with the OE splitter/no proportioning and a 1&1/8" alum dual MC & it stopped flawless and no excessive front pad wear. Pedal effort was fine (& I ain't that strong). the rears would lock up first on a panic stop but it rarely came to that if I kept my eyes off of the sidewalk.
     
  16. nm9stheham

    nm9stheham Well-Known Member

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    The rears tend to grab first due to the weight transferring to the front wheels in a stop; the fronts get more grip and can take more brake torque without locking up. The rears 'lose' weight momentarily during a stop and thus lock up more easily, so you have to reduce the line pressure to the rears OR make them somehow produce less braking torque. The smaller caliper bores do reduce the rear braking force, all else being equal. And so does making the coefficient of friction (CoF) of the rears pads lower than the front pads CoF.

    But whether it is enough reduction is the question. Ideally you would get about equal lock up front to rear (IF you are a decent driver!) over a broad range of pedal forces.
     
  17. goldduster318

    goldduster318 Overzealous Car Modifier

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    This is a big issue with rear drum brakes. Disc brakes are pretty much linear so as long as your piston size differential and rotor size makes sense, you shouldn't have much going on for locking the rear brakes first. For example, if you installed the complete Mustang Cobra kit from Dr Diff on your car, with a 15/16 master cyl, it should be pretty damn close.

    Fronts provide way more brake torque. Also, you are getting bias in a F/R split master cylinder.
     
  18. BillGrissom

    BillGrissom Well-Known Member

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    Can you elaborate? I did quick research and find no info that the MC itself is any different. What makes the system a diagonal or F/R split is the external plumbing. Link below gives some brake design info, including prop valve adjustment:
    www.sae.org/students/presentations/brakes_by_paul_s_gritt.pdf

    I found some guys questioning if some BMW's have a special "stepped MC", i.e. different bore size for the 2 pistons:
    www.bimmerforums.com/forum/showthread.php?1776935-Master-cylinder-(-diameter-
    But no reason to think that is true of the cloud car MC I used. It sure looks like the same bore size externally. Indeed, I have a spare MC rebuild kit for it, so I'll look at the piston again (don't recall but one diameter). Even if it were stepped, that would just affect the split to the R circuit and I have an adjustable prop valve so could tweak it perfect regardless.
     
  19. nm9stheham

    nm9stheham Well-Known Member

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    Not sure I understand this last part....?? Trying to figure out how the piston for the rears gets a different force applied on it than the piston for the fronts with the same bore size....
     
  20. goldduster318

    goldduster318 Overzealous Car Modifier

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    To put it simply, the primary circuit piston is the only one actually connected directly to the brake pedal. So, you push on the pedal, actuate the primary piston, which then builds some hydraulic pressure to actuate the secondary piston, at least until the initial spring force has been oer come. In a F/R split you'll find that the balance between the two return springs is altered to at least reduce the travel in the secondary piston vs the primary piston which therefore reduces pressure in the rear circuit and creates a bias, at least instantaneously during the apply due to hydraulic latency. This is probably enough to keep the rear brakes from locking first in a perfectly biased system. I know it sounds pretty minor.

    You may also find in some cases that the outlet ports have orifices which would be smaller for the rear brakes to reduce flow.

    The stepped piston master cylinders are pretty rare.

    Will you be able to get away with your cloud car master cylinder? Possibly. It may have some funky characteristics. You're welcome to try it. I'm not sure what you're using a 7/8 cylinder with for foundation brakes but it seems a little odd to me for the normal stuff used on these cars. Pedal travel is probably gonna be pretty long.

    At this point we do almost all the brake bias in the ABS/ESC module using both orifice sizing to slow the build rate at rear wheels vs the front and also by using electronic brake force distribution. Plus most stuff anymore is diagonal.
     
  21. Adam@Pachyderm

    Adam@Pachyderm Member

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    A late 90's Caravan master cylinder is very similar to what Wilwood sells and can get the job done for pennies on the dollar.
    Or spend the extra cash like I did on my A100 disc conversion.
     

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  22. nm9stheham

    nm9stheham Well-Known Member

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    OK, I agree on the rears being possibly delayed due to the springing differences; but my understanding of those springs is to make sure the secondary piston comes back to the right spot like the primary piston. not to provide any timing.. but maybe not?? The force out of the secondary piston will build up to the same as the primary in a fraction of a second once both fill ports are closed. The secondary piston could pass the fill port a little later than the primary piston, but any transient difference in fill port timing would not seem to be of any use as the car's weight is not yet shifted. And the primary cannot build to any real pressure as long as the secondary piston is not building pressure.

    So as far as any actual F/R brake bias, I am still not getting that with equal piston diameters....

    BTW, FWIW I think some GM MC's in the 70's/80's had stepped bore sizes......again IIRC. It has been a long time....
     
  23. nm9stheham

    nm9stheham Well-Known Member

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    And for another FWIW, my son and I are going to be taking one of the MC's you show OUT of his disc/drum system that was on the car when he got it. I suspect that since it is for disc/disc, it has no residual valve for a rear drum system. It is showing the signs: you drive for a while and put the brake pedal down and it goes a lot further to the floor than on subsequent pushes. (If you drive for like 5-10 seconds and put the pedal down, it does not do this. So it is not acting like air in the system.)

    Just for everyone to be aware......
     
  24. goldduster318

    goldduster318 Overzealous Car Modifier

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    On older cars, not really sure what they did. Maybe they are 50/50.

    On current stuff...well, they're showing the master cylinder bias on basically all of them, short of a few diagonal split cars. I work on ESC modules and we get this information direct from the OEMs. Of course we can't share this information directly. There are even diagonal split cars with not exactly 50/50 splits, which couldn't happen any other way. I believe that for a larger split they are also using orifices.

    Since the secondary piston floats, in order for it to move the pressure on the other side (being applied by the primary piston) must exceed both the pressure in the secondary bore AND the spring force of the secondary return spring in order to move. I'm not sure exactly how strong the springs are

    The only time the primary piston actually acts directly on the secondary piston is when the primary circuit has a leak.
     
  25. Adam@Pachyderm

    Adam@Pachyderm Member

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    Most Mopar disc brakes were 50/50 up until about 1982. It all started to change when fwd cars came into the mix and when the trucks started to get rear wheel abs to prevent them from locking the rear tires and spinning out. Ideally it's best to use something from a later model vehicle, can't beat technology.
     
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