JK Big Brake Kit on XJ

[Keith1138]

I'm currently in the process of building a jk 44 front axle to swap in a XJ. The XJ already has disc brakes in the rear because of the JK 44 rear axle that has been swapped in to it. The rear axle just has a set of stock disc brakes on it. For the front axle I'm considering doing a big brake kit since I need everything for the front brakes. My biggest question are;

To make it function safely and normally do I need a different proportioning valve?

Should I upgrade to the dodge Durango master cylinder that's bore is 1/8 of an inch bigger?

Why are the responses to questions like this on other forums so complicated and comical to read?

 

[Jody Treadway]

I'd source stock calipers and rotors. Your XJ will weigh less than a similar JK.
You won't need a different master. Just pop the oring out of the prop valve if you haven't already

 

[shawn]

X2. JK brakes are already a lot bigger than what came on the XJ, and work fine with stock booster and MC.

 

[Jody Treadway]

FWIW- I've installed a bunch of the "big brake kits" on JKs over the years. I really haven't seen a worthwhile improvement in braking. There's one on a JK in my shop right now. After driving it yesterday, it feels like every other pig ass JK out there.

 

[cranbiz]

With 37's, I know the big brakes, the larger master cylinder and booster do make a difference.

To me, it was noticeable, to others, maybe not.

 

[Bebop]

Why are the responses to questions like this on other forums so complicated and comical to read?

Because it requires actual research, understanding of braking systems and some due diligence.

And people that do the research don't usually share.

Better questions would be :

What is the pedal ratio of a XJ
What is the pedal ratio of a JK
What is the MC bore of a standard JK and the subsequent hydraulic ratio
What is the MC bore of an upgraded MC for the big brake kit in a JK and the subsequent hydraulic ratio
What is the hydraulic ratio of a durango MC with the big brakes kit

All these you could figure out by yourself fairly easily.

FWIW- I've installed a bunch of the "big brake kits" on JKs over the years. I really haven't seen a worthwhile improvement in braking. There's one on a JK in my shop right now. After driving it yesterday, it feels like every other pig ass JK out there.

Braking feel improvement is usually felt when the "bite" of the setup is increased.
Bigger brakes don't necessarily mean that.

Best things you can do for bite is softer/more aggressive pads, stiffer lines, lower clamping surface (paradoxically) and line pressure increase.
Going to "bigger brakes" usually does the opposite of all of this.

Good idea would be to transpose the complete braking system of a bigger, heavier vehicle to said JK, including pads, rotors, calipers, MC, booster, pedal ratio, and you'd definitely need a custom adjusted proportioning valve.
Basically it's a lot of work, and that's why nobody does it.

 

[Keith1138]

Thank you all for the replies.

I'd source stock calipers and rotors. Your XJ will weigh less than a similar JK.
You won't need a different master. Just pop the oring out of the prop valve if you haven't already

From factory a cherokee weighs 1200 pounds less than a 4 door wrangler. That is why I'm leaning towards a good factory set up for brakes might be the ticket.

Because it requires actual research, understanding of braking systems and some due diligence.

And people that do the research don't usually share.

Better questions would be :

What is the pedal ratio of a XJ
What is the pedal ratio of a JK
What is the MC bore of a standard JK and the subsequent hydraulic ratio
What is the MC bore of an upgraded MC for the big brake kit in a JK and the subsequent hydraulic ratio
What is the hydraulic ratio of a durango MC with the big brakes kit

All these you could figure out by yourself fairly easily.


Braking feel improvement is usually felt when the "bite" of the setup is increased.
Bigger brakes don't necessarily mean that.

Best things you can do for bite is softer/more aggressive pads, stiffer lines, lower clamping surface (paradoxically) and line pressure increase.
Going to "bigger brakes" usually does the opposite of all of this.

Good idea would be to transpose the complete braking system of a bigger, heavier vehicle to said JK, including pads, rotors, calipers, MC, booster, pedal ratio, and you'd definitely need a custom adjusted proportioning valve.
Basically it's a lot of work, and that's why nobody does it.

With removing the o-ring from the portioning valve what effect would that cause on the "bite" of the brakes. After doing research I've seen conflicting things about replacing the portioning valve in an xj with a v8 zj one. I've seen it is to fix your rear disc from locking up but also to fix if the brakes aren't working as well.

 

[Jody Treadway]

The main benefit to removing the oring vs buying a ZJ one is...it's a free test
Some of us salt and pepper bearded dudes remember the days of gutting a prop valve and 1986 E350 master cylinders fixing low pedal issues after a 4 wheel disc swap. We didn't do the same math that's available now. We were broke ass 20 somethings figuring out what works.
You're on the right path bud.

 

[Keith1138]

The main benefit to removing the oring vs buying a ZJ one is...it's a free test
Some of us salt and pepper bearded dudes remember the days of gutting a prop valve and 1986 E350 master cylinders fixing low pedal issues after a 4 wheel disc swap. We didn't do the same math that's available now. We were broke ass 20 somethings figuring out what works.
You're on the right path bud.

I thought y'all just tried different types of leather on the bottom of your crocks to see which one allowed you to stop better.

In all seriousness I'm going to try removing the o-ring before I mess with replacing any other components. I have also decided to go with just a decent set of oem size brakes.