WJ/KJ brakes in my XJ, poor rear braking.

I've had my KJ rear discs for a few years and I'm pretty sure they were working excellent at some point.

I recently took them off my 8.25 and stuck'm on my D44, but I'm not getting any power to them now. With the ass end lifted up on jacks, tranny in first and just idling, I can step on the brake with both feet and not lock up the rear.
When I bleed them, they seem to bleed just fine.

I've also recently put the WJ knuckles and Akebono dual piston on my front D30. They lock up without any hesitation on dry pavement, while the rears don't seem to do any thing. Was driving around in the snow/ice today and noticed that even on ice, with clutch disengaged, I can't lock up the rear.

That said, they lock up enough that I can't spin them by hand
in the shop.

Any idea's? I originally suspected that maybe the caliper fluid volume was just way more than the stock XJ master could provide, but tonnes of other guys seem to have no problem.

*edit* I did have to clock the rear calipers up a little higher than stock due to mismatched backplate bolt holes. Stock is clocked at exactly 90 degrees from top, while I think mine are clocked around 60 degrees. I keep thinking maybe the calipers don't bleed properly at that angle, but the pedal is firm. I'll try to bleed'm off the jeep again hopefully tonight, but I doubt that's goona do it.

Do you actually get fluid flow to the rear if you open a bleeder?

If you do, I would check and see if perhaps the pistons are seized.

Where is the bleeder valve in relation to the caliper on the rears? Did you screw with your prop valve?

i suspect you have the calipers on the wrong sides. the bleeder needs to be on the top.

Bleeders are at the top on both sides. Pistons are not seized and move freely in and out. When bleeding, seems like a normal amount of fluid being pushed out on each stroke.
Fought with this for 2 or 3 days before giving up and relying on the front brakes and t-case . In 4x4, I can lock up all 4, but only because I'm in PT 4x4 and the front brakes have enough stopping power to lock up all four wheels.

Up on the stand in 4wd, the brakes will stall the engine in 1st at about 1/4 brake pedal. In 2wd, I can step on it with both feet and the rear just keeps spinning under idle power.

I don't remember the details of how brake fluid is moved through the MC & prop valve in the XJ very well. There are a few different schemes that OEMs have used over the years.
What I think I recall, though, is that the XJ uses a conventional dual-piston master cylinder and a proportioning / differential combination valve.
The master cylinder actually has two pistons inside it. One is contacted by the booster rod, the other is floating. Under normal operation, the booster rod piston moves forward (sending fluid to the front brakes) and the second piston is shoved forward by that same pressure, since the fluid between the pistons doesn't compress. This second piston moves fluid to the rear brakes.
If that second piston isn't sealing to the MC bore any longer, you wouldn't be getting much pressure to the rear.
The prop valve (properly a combination valve) has two functions: one, is a brake pressure differnetial switch. Basically, it's a hydraulic contraption that trips a "brake" light on the dash when pressure isn't equal between front and rear brake circuits. The other is proportioning of pressure under heavy braking.
If your rear brakes are truly bled out, pistons move in / out freely, friction is relatively intact then all I've got is this : the secondary piston of the MC might be bad & it wouldn't be the first time I've seen a differential valve fail / act funny. It's been a few years, but I'm pretty sure mine didn't light up or do squat when I had a rear line blow up on me.

yossarian19 said:

I don't remember the details of how brake fluid is moved through the MC & prop valve in the XJ very well. There are a few different schemes that OEMs have used over the years.
What I think I recall, though, is that the XJ uses a conventional dual-piston master cylinder and a proportioning / differential combination valve.
The master cylinder actually has two pistons inside it. One is contacted by the booster rod, the other is floating. Under normal operation, the booster rod piston moves forward (sending fluid to the front brakes) and the second piston is shoved forward by that same pressure, since the fluid between the pistons doesn't compress. This second piston moves fluid to the rear brakes.
If that second piston isn't sealing to the MC bore any longer, you wouldn't be getting much pressure to the rear.
The prop valve (properly a combination valve) has two functions: one, is a brake pressure differnetial switch. Basically, it's a hydraulic contraption that trips a "brake" light on the dash when pressure isn't equal between front and rear brake circuits. The other is proportioning of pressure under heavy braking.
If your rear brakes are truly bled out, pistons move in / out freely, friction is relatively intact then all I've got is this : the secondary piston of the MC might be bad & it wouldn't be the first time I've seen a differential valve fail / act funny. It's been a few years, but I'm pretty sure mine didn't light up or do squat when I had a rear line blow up on me.

Click to expand...

This was my thought as well, but it just seems odd that it happens the second I swap out the old drums for the new discs.
I suppose I could always pick up another MC to test.

I did originally suspect that I may have got some air into the MC on the rear piston side, but I bench bled it and it seemed good. Could just be that I suck at bleeding MC's as well, but I can't imagine how I might have eff'd it up.

I did try both the XJ and the KJ prop valve, but as it's only used to decrease rear bias under hard braking, it shouldn't have any effect on the current issue (though it seems very few people have any idea what the prop valve does )

Use a stock XJ prop valve, but use the spring from a rear disk zj.

markw said:

Use a stock XJ prop valve, but use the spring from a rear disk zj.

Click to expand...

Tried that as well, but I suspect (no offense, just trying to educate) that you may be one of the majority of people who think the prop valve adjusts brake bias at all braking levels. This is not true though.
The prop valve only decreases rear bias under extreme braking conditions, in order to prevent the rear from locking up before the front. No matter what prop valve/spring you have in there, it won't have any effect on normal braking, and it's only function is to DECREASE rear braking under hard braking.

The reason guys swap the ZJ prop valve in with the rear disc swap is that on some rigs (depending on weight distribution) the rear discs will lock up way before the front. Swapping the prop valve or spring decreases rear bias under lockup conditions. Somehow though, tonnes of people do the swap for the exact opposite reason, and think it works. I suspect they think it works because they had weak rear brakes after the disc swap, did the prop valve swap and bled the MC at the same time. The MC had air in it which caused the original weak rear braking, and bleeding it fixed the problem.

I've tested this myself on another XJ.

Being that you now have dual piston calipers in front and discs in the rear, I'd use the questionable master cylinder as an excuse to go to a larger bore master cylinder to insure you are getting enough flow. There's a right up floating around about using a late 90's dodge 1.25" bore master iirc.

Try Pressure bleeding the brakes, I have seen some brake systems, where the caliper position was altered like you did, where normal bleeding & even using a vacuum bleeder did not get all the air out, Then after a pressure bleed everything worked correctly.
Good luck.

zachandandy said:

Being that you now have dual piston calipers in front and discs in the rear, I'd use the questionable master cylinder as an excuse to go to a larger bore master cylinder to insure you are getting enough flow. There's a right up floating around about using a late 90's dodge 1.25" bore master iirc.

Click to expand...

Why? The WJ, ZJ and XJ all have a 1" bore MC.

Talyn said:

Why? The WJ, ZJ and XJ all have a 1" bore MC.

Click to expand...

Right, and my xj master works fine with the huge brakes on my 1 ton axles. Also worked fine with my wj/rear disk setup i had before.

It's a matter of preference to some degree. A 1" bore is going to have more braking power but lower in the pedal stroke. Some folks like a quicker engagement, which means more fluid volume pushed per degree (or inch, or whatever) of pedal travel, which means a larger MC.
I think for WJ / ZJ kit, the 1 1/8" Durango master cylinder is preferred. Dana 60s, go Ram 2500.
Or keep it stock. Whatever.

I do prefer a more sensitive brake pedal than most, but this isn't about preference at this point . It's a complete inability of the rear brakes to lock up under simple braking conditions like on ice, or being able to stall the motor when up on stands.

I'll try to get under it today to do another bleed with different caliper angles.

Finally got around to bleeding the rears again. Took the calipers off the mounts and bled again with the caliper clocked at 90 degrees. No change at all, but does seem like there's not a lot of fluid coming out of the rears.

I agree with the folk that say the WJ/XJ/KJ all use the same 1" bore MC, so there should be more than sufficient fluid volume in there, unless perhaps the stroke of the rear piston is dramatically shorter than the front. This may be the case, but still, this isn't an uncommon swap. Most guys use the ZJ/WJ calipers, but they look to be about the same fluid volume as the KJ's. I can't imagine this is an issue of having the right components. Something must be broken.

That said, a kinked brake line shouldn't be the issue as it would only cause slow pressure build up, but I'm holing the brake pedal down for up to a minute to make sure the pressure is stable. The rear just isn't getting the volume it needs, while the front, even with the big akebono calipers, has no problem locking up my 33" on dry pavement.

I had thought the problem wasn't as big an issue earlier, because I'd tested the brakes on some big muddy hills, and they were in fact, all locking up just fine, but it didn't occur to me till now, that I was always in 4x4 when I got lock up. The front calipers are actually braking the rear wheels through the t-case So, that in mind, I drove down the road in 4x4 and voila, all 4 wheels lock right up. Of course, this doesn't work in 2wd and I can only imagine how good my braking would be if I had the rear calipers both helping.

Any ideas? Maybe my MC was just borked to begin with and I didn't notice?
If that's the case, what could break, in the MC, to cause low volume to rear but normal volume to front? Floating piston stuck maybe, but you'd think that woudl eventually lead to the piston getting destroyed and my brakes equalizing.

blistovmhz said:

Tried that as well, but I suspect (no offense, just trying to educate) that you may be one of the majority of people who think the prop valve adjusts brake bias at all braking levels. This is not true though.
The prop valve only decreases rear bias under extreme braking conditions, in order to prevent the rear from locking up before the front. No matter what prop valve/spring you have in there, it won't have any effect on normal braking, and it's only function is to DECREASE rear braking under hard braking.

The reason guys swap the ZJ prop valve in with the rear disc swap is that on some rigs (depending on weight distribution) the rear discs will lock up way before the front. Swapping the prop valve or spring decreases rear bias under lockup conditions. Somehow though, tonnes of people do the swap for the exact opposite reason, and think it works. I suspect they think it works because they had weak rear brakes after the disc swap, did the prop valve swap and bled the MC at the same time. The MC had air in it which caused the original weak rear braking, and bleeding it fixed the problem.
I've tested this myself on another XJ.

Click to expand...

Factory proportioning valves have a breakover or "knee" point where they increase pressure in proportion to the front and at a certain point they reach the breakover point and decreases the amount of pressure. I only swap the spring, not the valve, way easier to do, and the spring determines the breakover point.

You'll also notice the factory does a ratio on the piston diameters front to rear.

There's volumes on this stuff and why it works, and math to figure out what sizes of master cylinders to use with caliper piston diameters.

Most guys do the brake swap and don't mess with the prop valve. Or they put those chevy front pistons on the rear and wonder why they lock up the rears, or the just pull the guts out of the prop valve.

How's the alignment of the disc in the caliper?

My setup is the Explorer rear discs, and WJ fronts, stock mc, booster and ZJ disc brake spring in the prop valve. I can do hard braking on dry pavement and it stops without locking up the rears, and it stops now. I'm on 33's.

Crazy thought - if you have crap (that's a technical term) built up in the MC's secondary cylinder orifice (fluid passage between "rear" portion of MC and prop valve", that could be the culprit as well.
You'd have to re-bleed the MC after, but you could remove the lines from the MC and have an assistant push the pedal down, see if you get an even fluid volume between the two MC outlets. Attach the primary cylinder line (better yet, plug it - Oreilley's shoudl have the fitting) and try again - see if the secondary cylinder is moving fluid good and proper.
It'd be good to eliminate as many suspects as possible by experimenting before buying parts, y'know?

yossarian19 said:

Crazy thought - if you have crap (that's a technical term) built up in the MC's secondary cylinder orifice (fluid passage between "rear" portion of MC and prop valve", that could be the culprit as well.
You'd have to re-bleed the MC after, but you could remove the lines from the MC and have an assistant push the pedal down, see if you get an even fluid volume between the two MC outlets. Attach the primary cylinder line (better yet, plug it - Oreilley's shoudl have the fitting) and try again - see if the secondary cylinder is moving fluid good and proper.
It'd be good to eliminate as many suspects as possible by experimenting before buying parts, y'know?

Click to expand...

Yup. I don't like throwing money at a problem unless I'm confident it'll solve it.
I had a thought. I took the rear pads off and stuck a u-clamp on the piston and hit the brakes to see if it'd bend the u-clamp. Eyup!
So I think the rears may actually be working fine, but the pads are just really junk but aren't showing it.
Going to throw on some new pads and maybe get the discs turned.

I usually don't even bother with turning rotors... just toss new ones on. Does rockauto ship to Canada? They carry KJ rotors for $17 each it looks like.