Write-Up 8.25" Rear Drum to ZJ Disc Brake conversion

[WB9YZU]

So here's the back story.
After wheeling XJs for years, I go and pull the 01' out of the garage the 2nd time after WF and find that the rear brakes do... nothing. Turns out that the cylinders are rusted in their bores. You know the game, play with springs getting the shoes off, play with cylinders, play with springs again putting them back on. Not all that much fun.

The opportunity presented itself shortly afterwards to pickup some late 90's ZJ backing plates with all the parking brake hardware, calipers, pads, and hoses. I couldn't say "No" to that.

This is an easy mod if you have about $200 laying around that you don't mind spending and want to spend time working on your Jeep with a decent return on your investment.

Parts:

If you look in a JY you will be looking for this stuff from a late 90's V8 Grand Cherokee:
*Rear Disc Backing plates.
*Right and Left brake hoses
*Calipers (& pads if in decent shape)
*Caliper bolts
*parking brake hardware (Everything inside the drum part of the disc.
*Discs (or will cost you $30 a piece at NAPA)

Other things you will need:
* 2 feet of 3/16" brake lines and 2 extra ends.
* 2 brake fitting unions.
* Brake Fluid
* 2qts Axle lube (If you have a limited slip, get additive also)
* 10 Front wheel studs for a 97 and up XJ.
* 8 3/8"X1.5" Grade 8 bolts and Nuts.
* Torch (For those annoying brake line fittings)
* Penetrating Oil (For same)

Procedure:

* Jack up the rear of the Jeep, put it on stands and remove the wheels.
* Remove rear cover to drain fluid.
* Remove bolt retaining cross shaft.
* Remove cross shaft (Hint! Do NOT rotate anything after removing shaft. You should have thought far enough ahead so you can see the C-Clips you will be removing)
* Push each axle IN and remove the C-Clip from the axle inside the carrier.
* When C-Clips are removed, pull each axle straight out and set aside.
* Re-install cross shaft. This keeps the gears in-place.

* Take this opportunity to replace the wheel studs on each axle flange with the 1 15/16" wheel studs you picked up at the parts store.
* To remove them, I stood the axle on a piece of cardboard and, using a 5lb hammer, struck each stud squarely until it fell out.
* To re-install, I lubricated the shank of the wheel stud and hammered them back into the flange. You could also use a ball joint press for this with good results.

* Remove old brake hardware and backing plates. Don't trash the hardline or you will end up replacing it. Don't trash the E-Brake cable or you will end up replacing it. Punch out the old studs that hold the backing plate. They are too short.

Should look like this when you are done with this step:

* Using a grinding stone or a rotary file, remove material from the inside of the new backing plate where it fits over the axle. You will end up talking off about 1/8".

* Find your (8) Grade 8 3/8" X 1.5" bolts and Grade 8 nuts.
* Slide backing plate over the axle with the caliper mount facing towards the rear. Bolt it on with aforementioned bolts.

* Assemble parking brake shoes and hardware. ( I figure you don't need a picture for this )

* Install parking brake Cable. To get the XJ end to work, I took a pliers, grabbed the ball end end and bent the cable 90 deg. Then I struggle like heck to get the spring pushed up on the cable while I pushed the cable onto the slot on the e-brake end. To be honest, it's a bit of a chore, but from my perspective it's no different than putting a E-Brake cable on XJ hardware

* Grease the seal surfaces on the shafts and insert them in the tube.
* Remove Cross shaft, push axles in, install C-Clips and reinstall pin.
* Put the Cross shaft retaining bolt back in (Or you will be sorry )
* Clean, and RTV the cover and install it.
* Put 2 qts of your fav gear lube in. If you have a LSD, don't forget the additive!

* Install the disc (You might as well take a few moments to adjust up the parking brake now as it is accessible.)

* Figure out how you are going to attach the hard line to the hose. I used a short piece of 3/16" line with ends and a union to do the job. I also drilled and tapped the E-Brake cable holder to accept a 1/4-20 bolt to hold the hose in place. Notice the short U shaped adapter and union I used to connect the existing hard line to the brake hose.

* Load the caliper if it isn't already loaded.
* Put anti-seize on the caliper bolts and slide them into the caliper.
* Install the caliper on backing plate. (Hint!) The bleeder goes on the top side!
* Attach the brake hose to the caliper, with a copper washer on each side of the hose connection, using the hollow connecting bolt that came with it.

* Bleed brakes while ever thing is open for inspection. I used a cheap vacuum bleeder.

* Install wheels and torque to 90 Ft/Lbs.

* Verify the brakes actually work before heading off.
* Verify parking brake works.
* Adjust parking brake shoes as needed. There is a slot behind the adjuster wheel for this.

Results:

Once I got the parking brake shoes adjusted and re-bled the brakes using the partner inside the Jeep method, they work well. The Jeep stops easier and the E-Brake is actually functional 100% of the time. :thumbup:

 

[themauler]

Not 100% sure but someone else may correct me, You aren't going to utilize 100% of the rear brake power until you get the proportioning valve from the same jeep as long as it fits. I did this swap with d35 discs and with brand new brake pads and discs I still can't lock the rear wheels (no abs). I will never know until I get a proportioning valve from that same year GC

 

[yossarian19]

Sticky.
Great writeup.

 

[WB9YZU]

Sticky.
Great writeup.

Thanks I'll do a PDF for the FAQ Archives later.

 

[xpedition xj-eh!]

Not 100% sure but someone else may correct me, You aren't going to utilize 100% of the rear brake power until you get the proportioning valve from the same jeep as long as it fits. I did this swap with d35 discs and with brand new brake pads and discs I still can't lock the rear wheels (no abs). I will never know until I get a proportioning valve from that same year GC

I have GC discs on my 8.25. I don't have much to add other than my own non-scientific experience. I briefly drove it with the stock proportioning valve and it seemed "OK". I didn't notice anything scary about using the stock XJ valve but I didn't really "step on it" to find out either. Shortly after, I got around to installing the GC valve that came from the same donor vehicle. It's been like this for over a year and the brakes work as one would expect. They lock up at roughly the same time on gravel. I've only stomped on the brakes a couple times on pavement and all I can say is, I did not notice the rears locking up before the fronts. All in all, I'm very happy with the upgrade mainly because I hate fiddling with temperamental drums. If you are running an XJ prop valve and are considering putting in a GC valve, then I would suggest you take your XJ for a rip, hit the brakes hard a few times on varying surfaces. Make sure your rears are capable of locking up, and that they do it after the fronts lock up. If that is the case, then I'm guessing you are fine as is.

 

[WB9YZU]

I looked into the ZJ proportioning valve. There are folks who went both ways, so to be honest, I went with the "less work" method

But a word about proportioning:
The stock bias is towards the front brakes on a XJ (Actually on all cars), so the fronts will always lock up before the rears.

You don't want the rear brakes to lock before the fronts; it's all about predicable handling.

 

[xpedition xj-eh!]

a word about proportioning:
The stock bias is towards the front brakes on a XJ (Actually on all cars), so the fronts will always lock up before the rears.

You don't want the rear brakes to lock before the fronts; it's all about predicable handling.

Yep, that's what I meant above when I said "Make sure your rears are capable of locking up, and that they do it after the fronts lock up"

In addition to the handling aspect, it has to do with the fact that when you brake, the weight of your vehicle dives forward and places more weight on your front wheels.. thereby making them capable of more grip.. therefore they have greater stopping power.

 

[yossarian19]

I think I heard, somewhere, that modern vehicles are supposed to lock the rear first - so you don't lose steering when the fronts slide under heavy braking.
Too lazy to google that.