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Moses Ludel

Rear Disc Brake Conversions, Master Cylinder Needs and the Drum Brake Residual Pressure Valves

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Disc brake conversions are popular, and I cover that topic in my Jeep CJ Rebuilder's Manuals (1946-71 and 1972-86 Editions, Bentley Publishers).  Whether the CJ has a four-drum system or a disc front/drum rear system, the master cylinder must be considered during a disc brake conversion.


There are two master cylinder concerns when converting to disc brakes: 1) the piston bore size and fluid volume per stroke of the pedal and 2) any "residual valves" that might have been used for the drum brakes.  For disc brakes to work, the master cylinder must have enough fluid displacement to apply the calipers and pads.  Disc calipers use more brake fluid per pedal stroke than properly adjusted drum brakes.


If the Jeep is a vintage CJ 4x4 with a single master cylinder and drum brakes, especially the 9-inch diameter drum system, the stock master cylinder will be inadequate for modern disc brake calipers.  Drum or disc brakes, I'd want to get rid of the single master cylinder for safety sake, regardless! 


In converting to disc brakes, the best choice here should be a modern four-wheel disc brake type dual master cylinder retrofit.  A retrofit can even be done using the original, through-the-floor brake pedal, as I illustrate in the 1946-71 Jeep CJ Rebuilder's Manual (Bentley Publishers).  I fabricated a safe, sturdy mount for a later dual master cylinder—mounted beneath the floorboard like the stock master cylinder and actuated by the stock brake pedal.


Sometimes, a disc/drum master cylinder will have adequate fluid displacement on the rear drum circuit to operate retrofit disc rear brakes.  Again, this depends on the master cylinder's bore size and stroke per pedal application.  The rear fluid reservoir is often smaller, so keep fluid at the recommended full level.


On 1972-up Jeep CJs with four-wheel drum or disc front/drum rear brakes, you may be able to use the stock master cylinder with a disc brake conversion.  Be aware, though, that some master cylinders will require removal of the residual valve(s) from the master cylinder ports.


The "residual valve" is important on many drum brake systems.  To keep the wheel cylinder cup lips expanded, which prevents fluid seepage from the wheel cylinder with the brakes released, a valve is built into the hydraulic system to hold "residual pressure" in the wheel cylinders when the brakes are released.  Early single master cylinders and many four-wheel drum or disc/drum dual master cylinders have built-in "check" or "residual" valves.


This residual pressure is below the tension of the brake shoe return springs.  Residual pressure is simply to keep the wheel cylinders from drawing air or leaking fluid when the brakes are released.  This pressure is typically around 12 PSI, well below brake shoe return spring tension.


By design, disc brake calipers do not require residual pressure.  The pads release pressure with the pedal release.  There is adequate fluid available in the circuit to apply the brakes without lag or hesitation. 


Some disc brake hydraulic systems, do have very slight residual pressure to keep the pads close to the rotors at all times and improve brake response time during pedal application.  This pressure would be around 2 PSI and not enough to cause premature pad wear, fade or overheated rotors.


Note: If you're using a four-wheel drum or disc/drum CJ master cylinder, check the fluid line ports for a residual valve.  Typically, this valve is simply a rubber plunger and balance spring at the back side of the tubing flare nut seat.  With the brake lines removed from the master cylinder, you can see the rubber plunger through the passageway at the center of the tubing flare nut seat.  This seat is removable for service and seat replacement.  If you are curious how to safely remove the seat, I'd be happy to detail—ask here at the forum!


Caution: When retrofitting from drum to disc brakes, you need to remove the drum brake residual valve(s).  Earlier Jeep dual master cylinders for four-wheel drum brakes have residual valves at both the front and rear fluid line ports.  OEM disc/drum brake systems can have a residual valve on the rear brake circuit.  If the residual valve for drum brakes is left in place, the disc brake pads will drag on the rotors with the brake pedal released.  This can cause excessive pad wear, brake fade and even wheel lockup.


One disc brake conversion example is our fellow forum member "LastCJ7".  He has a 1986 CJ-7 Jeep (disc front/drum rear factory brakes) and is converting to rear disc brakes.  He's trying the CJ-7 dual master cylinder before considering a late Jeep TJ Wrangler Rubicon (four-wheel disc from the factory) master cylinder...LastCJ7 needs to make sure there is no residual valve holding pressure in the rear brake system with the brake pedal released.


On later disc/drum master cylinders, there may not be a residual valve in the rear brake circuit.  Many manufacturers have changed over to stiffer wheel cylinder cup expander springs with sturdier cup expanders.  This measure keeps the rubber cups expanded with the brakes released and serves the same purpose as older residual valve systems.


When converting to disc brakes, explore whether your original dual master cylinder uses a residual valve or valves.  Vintage, single master cylinders have a check valve within the master cylinder to hold residual pressure in the system—one more reason why a single master cylinder is not a candidate for a disc brake conversion!


Make sure the master cylinder's fluid displacement (per pedal stroke) will meet disc brake caliper requirements.  If in doubt, retrofit a combination valve and master cylinder from a similar chassis—like retrofitting a Jeep TJ Wrangler Rubicon master cylinder and combination valve to a CJ-7 chassis.


Summing up, make sure the brake hydraulic system is compatible with the disc brake calipers and rotors.  Both the CJ-7 and TJ Rubicon are on a 94" wheelbase, each has beam axles and an inline six-cylinder engine, their curb weight is a close match, so they should have similar braking needs and characteristics...Jeep TJ Wrangler Rubicon brake components would be a good template for the CJ-7 wheelbase and four-wheel disc brakes.



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Very informative.  Someday I hope to convert to disc rear brakes, however, my rear axle is a Ford Sterling 10.25".  Kits are out there but far beyond my budget.  I'd sure like to ditch the heavy drum that on muddy conditions tends to collect grime (when dries out is like cement).  In winter, moisture in the drums will freeze up making mobility impossible.


One challenge is I demand an e-brake on my 4x4 for many sane reasons and the kits offer that feature for even greater cost.  So for now, will stay with drums and hope all that unsprung weight keeps me closer to terra firma, LOL.

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Thanks, I thought this info might be useful, folks often mix up parts in an effort to save cost and avoid expensive disc brake conversion "kit" approaches...


I got the visual on "cement" mud around the huge brake drums or slushy freeze-ups in the winter!  Those massive rear drum brakes on a CJ-8 Jeep Scrambler should be overkill, for sure.  If you're using the stock master cylinder, there is likely less rear brake apply pressure than with a Ford E-van master cylinder.  You can compare master cylinder bore diameters to get a better sense for this.


There's also the concern around too much braking capacity at the rear.  Your 10.25" Sterling axle is designed for a high gross weight application (E-van), and you might find that OEM discs for that kind of rear axle would provide too much braking force.  The front axle and disc brakes are 3/4-ton capacity truck, right?


When the time comes, we can kick around details on braking capacities for available front and rear brakes.  What you want to avoid is "over-braking" at the rear, as rear lockup or bias can cause a vehicle to spin out, especially on loose or slick surfaces at speed.  A classic stunt driving trick is to apply the rear parking/emergency brake on a very slick surface, without touching the brake pedal.  The effect is dramatic:  The vehicle immediately spins around.  (Caution: Do not attempt this trick unless you're on a professional, slick skid pad.  Uncontrolled, it can be very dangerous and even cause a vehicle rollover.)


Spinout can occur with an over-braking bias toward the rear of the vehicle.  Driving on an icy highway or slick trail, especially a downgrade, could be a recipe for disaster if you need to stop quickly.  One way to offset this bias is use of a manual brake proportioning or metering valve, adjusted to reduce brake apply pressure at the rear. 


Some trucks even incorporate an OEM manual brake proportioning valve at the rear axle.  Activated by a mechanical lever arm, the valve reduces brake apply pressure when the vehicle's rear end gets light (springs extend) during hard braking with front end dive.  We had K2500 GMC Suburban 4x4s from the mid- to late-'80s that had this feature... 


We can discuss all of this further...Have a safe, fun 4th of July RareCJ8!



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On the CJ8 Scrambler, I am using the stock proportioning valve and the vacuum assist is a Navajo Booster sold by Harold Off in NM.  Works great.  I have tested the panic stop and all is within proper spec front to rear ratio.



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On the shorter wheelbase Jeep CJs, brake proportioning seems less touchy.  Your CJ8 Scrambler is 103.4" wheelbase (stock), so the 4x4 is a candidate for more accurate brake proportioning, front to rear.  Glad the large drums brakes work this well with the disc front brakes.  Your Jeep's modified curb weight is considerable, my guess is around 4,800-5,200 pounds?


Thanks for sharing these details.  Other trail runners may make similar axle swaps to their CJs.  For the sake of fellow members and CJ builders, can you share the origins (model and year) of the front and rear retrofit axles and whether the brakes are stock for each axle?  I'm guessing that you're also using the stock CJ-7 master cylinder?  The axles look like their 3/4-ton truck track widths, what size tires are you running?  What's the approximate lift?  I know you've modified the springs considerably.


For forum members interested in seeing this CJ-8/Scrambler up close with its fresh 4.6L Hewes Performance stroker motor, here is a link to the magazine's HD video walk around of the Jeep and Mark's narrative: http://www.4wdmechanix.com/HD-Videos-Jeep-CJ-8-4.6L-Stroker-Power.html.  We featured this Jeep because it's the "real deal", a tough, no holds barred trail runner that constantly plies northern Nevada's back country and the toughest Sierra trails. 



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