Disc Brake Conversion... Doing It Right?

[NotMatt]

So here's something I've been mulling over in my head for a bit here.

 

I have an 89 comanche with a drum brake 35 in the rear. I have an XJ balance valve installed and one line running to the rear brakes, instead of the funky factory level prop valve setup. The rear brakes have been nothing but trouble. They grab and lock the rear tires occasionally when they are damp, or during hard braking. And now the brake light has started to come on when I step on the brakes hard, indicating that the rear brakes are leaking or need adjustment (balance is out between front and rear).

 

So, given the crappiness of the rear drums and the crappiness of the rear model 35, I'm planning on finding an explorer 8.8 with discs to put in the back. That part I have covered, fabrication-wise... the actual disc conversion is easy

 

My question is, if you were doing this, and wanted to mimic a factory setup as close as possible, how would you do it? Obviously the master cylinder and balance valve on the XJ/MJ was not designed to have rear discs. I've heard people talk about the rear brakes over powering the front brakes during hard stops after disc brake swaps. This is absolutely the worst scenario, especially for winter driving, with a lightweight rig like the comanche.

 

So, there's talk of installing an adjustable proportioning valve for the rear brakes... maybe a residual valve (I am uncertain of how this part actually works or what it's purpose is)... should I put it back to the dual line height based prop valve that came factory? Should I replace the entire balance valve assembly and/or master cylinder with one from a rig that came factory with disc brakes? If so, which rig would work the best? Maybe Jeep Liberty or JK?

 

I want this to be a safe, good performing setup that works like it would have came that way from the factory. Any input?

[Blue88Comanche]

you can swap to a 8.25 from a Cherokee then use the disc brakes from a KJ. you might be able to use the whole axle just rework the spring perches. there was a write up somewhere, it was more or less remove the drum hardware, and replace with the disc hardware. i don't remember what he was using for prop valves and the parking brake cable.

 

I dout you would be able to use a prop valve from a KJ as they have ABS, but i have not looked at the at anything other than our 03 and 05.

[Backdraft]

I have a 2001 XJ prop valve in mine and run a rear D44 with a ZJ disc brake conversion. I haven't had any issues with the rear locking up or bad braking. Bear in mind I'm running 33x12.50 BFG KM2s on it which may contribute to the rears not locking. Also, I don't see much snow or ice either. I can tell you that I don't have any funky issues with mine and am very pleased at how well it stops.

 

Edit: what Jeff suggests with the 8.25 and KJ brakes is what I'm doing with my sons MJ.

[willi2ds]

I'm running a 8.25 with kj discs, I eliminated the 2nd line and put an adjustable prop valve in line, I have 35's and it stops great

[mnkyboy]

I have a D44 with D30 disc brakes,I used a later XJ dual diaphram booster and prop valve.

[Abyx]

I have an 8.25 with Grand Cherokee D35 brakes; works just fine. I don't even HAVE a preportioning valve; they can lock up my 35s if I want them too.

 

 

Why exactly does 'everyone' reccomend KJ brakes for the 8.25? What's the difference??

[Blue88Comanche]

KJ rear disc brakes are a direct dolt on for Older 8.25 axles. it's just the easiest conversion if you have an 8.25 already. I plan to swap Dads D35 for an 8.25 for that reason

[NotMatt]

Thanks for the replies guys, gives me something to think about. I know my options on what I can do to get the actual discs in the back, my main concern was the lines and master cylinder and any additional valves or replacement prop valves that I should look at.

 

I think right now what I will do is go ahead and swap the discs in with the stock master cylinder and stock XJ prop/balance valve as it is and see how it works. I'm so tired of the stupid drums that don't auto-adjust right and apparently now I've developed a leaky wheel cylinder somewhere as I checked the master last night and the resevoir for the rear brakes was completely empty. Probably why the brake light was turning on when braking. :|

[johnnyc]

I'm using a prop valve from a 4-wheel disc brake ZJ.

[Blue XJ]

If you have the Dana 35 your best bet would be to find a grand Cherokee (zj) with rear discs. They had Dana 35's as well so they are a direct swap, grab the trip valve too and your good to go. That's how I did mine, except I had to modify the mounting plate to fit on an 8.25.

[HOrnbrod]

I'm running a 8.25 with kj discs, I eliminated the 2nd line and put an adjustable prop valve in line, I have 35's and it stops great

 

So what you're saying is you replaced the load sensing valve with a manual proportioning valve then disconnected the 2nd "emergency" line from the rear axle brake hose and the front distribution block and plugged both ends?

[willi2ds]

So what you're saying is you replaced the load sensing valve with a manual proportioning valve then disconnected the 2nd "emergency" line from the rear axle brake hose and the front distribution block and plugged both ends?

 

http://www.flickr.com/photos/67584637@N03/8034603692/

Yeah pretty much, I put a bolt in the distro block to plug it and the other end was just unhooked from the original valve. Here's a pic of where I mounted my Adj. Prop Valve. I can just reach in from the wheel well and adj. as necessary. I love it

[skidoo_j]

http://www.flickr.co...N03/8034603692/

Yeah pretty much, I put a bolt in the distro block to plug it and the other end was just unhooked from the original valve. Here's a pic of where I mounted my Adj. Prop Valve. I can just reach in from the wheel well and adj. as necessary. I love it

 

I was thinking of doing this too, but keeping the emergency line and tee after manual valve.

[HOrnbrod]

http://www.flickr.co...N03/8034603692/

Yeah pretty much, I put a bolt in the distro block to plug it and the other end was just unhooked from the original valve. Here's a pic of where I mounted my Adj. Prop Valve. I can just reach in from the wheel well and adj. as necessary. I love it

Yes, that looks pretty good. Kind of what I had in mind. Thanks for linking the pics. :cheers:

I was thinking of doing this too, but keeping the emergency line and tee after manual valve.

 

That setup would be a brake bleeding nightmare.

[skidoo_j]

 

 

That setup would be a brake bleeding nightmare.

 

How would it be any different then bleeding a stock setup?

[HOrnbrod]

I was thinking of doing this too, but keeping the emergency line and tee after manual valve.

The emergency line goes from the bottom front port of the distribution block to the T fitting on the axle, bypassing the load sensing valve. That line is the bear to bleed, at least it was for me. I don;t follow what you mean by your statement in bold above. After?

[skidoo_j]

The emergency line goes from the bottom front port of the distribution block to the T fitting on the axle, bypassing the load sensing valve. That line is the bear to bleed, at least it was for me. I don;t follow what you mean by your statement in bold above. After?

Oh meaning that the emergency line would connect after the manual valve. IIRC that's the stock setup. So pretty much I'd just want to replace the auto valve with a manual turn valve. I've been holding off as I'm watching for a dual vacuum booster on sale, and then I'll probably have to run new lines anyways.