fannitti

I wanna do kj rear disc swap on my 89 xj. If I swap a proportioning valve from a zj also. Can I use it with my older style master cyl?

bad_idea

I just put an Explorer 8.8 in the back of my Cherokee. It has disc brakes on it. The only thing I did was hook up the lines. Didn't touch the prop valve. I haven't driven it much yet. I took it for a spin and the brakes seem fine. I slammed on the brakes at ~35 mph and it stopped smoothly.

In the reading I have done, it is common to pull the internals out of the ZJ prop valve and install them in the XJ prop valve. I plan to replace the master/booster soon to update to the 95 booster (better stopping due to higher line pressure) and will mess with the prop valve then. Until then, everything I have read says the brakes perform fine with the stock XJ prop valve. I have a 90 btw.

cruiser54

Get your stuff from BigDavid here on CF.
You'll get the correct prop valve guts also.

cruiser54

You will LOVE the big booster.

bad_idea

That's what I've read. My other car is a 2002 z28. Going from the Camaro to the jeep is a little unsettling as far as brakes go.

winginit

The stock Cherokee master cylinder has a 1″ bore and would probably work alright .
But you should change to a master cylinder with an 1 1/8 bore like a Durango or Dakota that came with 4 wheel disk brakes .

cruiser54

And what kind of practical experience do you possess doing these brake mods? Like first hand.....

Or, are you just throwing crap out there to muddy the waters?

winginit

Well sir I am 56 years old been a mechanic all my life , I work for NEV-department of transportation as a mechanic for the last 17 years built a few drag cars over the years just for fun !!
I am not trying to muddy the waters , rather I am trying to be helpful .

bad_idea

Damn cruiser. You don't pull any punches do you? Valid question though. What is the added value of the larger bore? Will it bolt to the Cherokee booster? I'll be swapping out the booster and master anyways. Not a big deal to change the master for a different one.

winginit

I dont know about early XJ's but 97 -01 it should bolt right up hood clearance will be close .
It is important to consider that the smaller cylinder makes more pressure but the smaller bore will move less fluid. More travel will be needed to make up for the reduction in fluid moved IE a low brake pedal .
The engineers have done the math for us , 4 wheel disk's on Durango's and Dakota's have 1 1/8 master cylinder . will the 1" bore work , yes it will .

cruiser54

Where do I start?

I have done ZJ rear disc swap with stock single diaphragm booster, 95 to 96 booster upgrade with drums, 99 to 04 booster and master upgrade, and 99 to 04 booster and master upgrade with ZJ rear discs, 99 to 04 WJ booster master upgrade with ZJ discs with and without swapping the prop valve guts.

I know how this stuff works.

To have someone chime in "being helpful" and misleading folks and possibly sending them down a rabbit trail which could cause them to spend time, money ad energy on speculation goes against my grain.

If folks aren't held accountable, what's the result for the end user?
As a moderator I have a duty to our members.

cruiser54

Muddying the waters in favor of relying on proven methods.

Sorry, but it's true.

Basslicks

I think the main thing we're trying to figure out here is, have you actually done this yourself, have you compared results of before and after the swap, and if so, is there any real gain versus the cost and effort involved?

Most people have noted being able to lock up the wheels with 33" tires after swapping in a dual-diaphragm booster.... that's without swapping a single other component and without nearly as much braking effort. That's contrasted against the OEM single-diaphragm on earlier XJs (such as mine) with the same size tires (Or even less - mine has worn-out 32s) and STILL not being able to lock up the wheels with every bit of braking force you can muster through your leg.

Theory is one thing. Results are king.

bad_idea

So... I have a 90 with the 8.8 in the back (disc brakes). Booster is shot. Plan to go to Advance Auto and buy new booster and mc. What should I buy. Specifically. Year, make and model. Will then hit the scrap yard for a proportioning valve. From my reading, I am looking for the guts to a ZJ prop valve for the disc brakes. But, since I am swapping the master to the newer one, would it be easier to grab the prop valve and lines to master from the ZJ? Are the fittings on the ZJ master the same as the mc I plan to install? Will the outlet side of the prop valve connect to my lines running to the wheels? I would like to avoid flaring lines if I can.

I know this is a mess of questions sidetracking this thread. I apologize if this is overly rude, but it seems to relate to OP question to me.

cruiser54

Get a 99 to 2004 WJ master and booster from the junkyard with the lines attached.

Snag the prop valve guts from a 94 to 98 ZJ.