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coloradoinsane
April 14th, 2013, 19:12
Hey guys. Did a disc brake conversion on my 2000 XJ this week. Everything went smooth until I drove it the first time. Pedal was pretty spongey for my liking. And no, there wasn't any air in the system (bled the crap outa it). From all that I've read the stock MC (master cylinder) doesn't move enough volume for front and rear discs. (Waggy dana 44 single piston calipers up front, el dorado rear calipers) from what I had read the 98' and up MC was a direct fit so I swapped out the MC and put the longer push rod (bolt with head ground off) on. All went smooth again except now the pedal is too darn firm, can barely lock up the brakes on pavement going 10 mph (and that's nearly standing on the pedal). I checked the brake booster-vacuum line is still connected with not leaks. Any suggestions?

yossarian19
April 14th, 2013, 21:27
Never heard of the MCs getting swapped between years of XJ. I'd thought they were all the same after the Renix years.

Pedal too firm, if the hydraulic component sizes haven't changed, has got to be a booster problem. If you went to a 1.25 or 1.37 bore master cylinder, I'd say it was that...

coloradoinsane
April 15th, 2013, 06:12
Oops!

coloradoinsane
April 15th, 2013, 06:12
Never heard of the MCs getting swapped between years of XJ. I'd thought they were all the same after the Renix years.

Pedal too firm, if the hydraulic component sizes haven't changed, has got to be a booster problem. If you went to a 1.25 or 1.37 bore master cylinder, I'd say it was that...

Oops! Sorry guys, I meant to say from a 98 dodge 2500 (1.25 bore, was under the impression that it would move the needed amount of fluid). Kids were distracting me while I was typing it up!

XJLI
April 15th, 2013, 06:19
Oops! Sorry guys, I meant to say from a 98 dodge 2500 (1.25 bore, was under the impression that it would move the needed amount of fluid). Kids were distracting me while I was typing it up!

you swapped the dodge master? that thing is way too big for those brakes. stock (97+) MC/BB works great for the brakes you're running, my buddy had the same setup and it felt awesome.

coloradoinsane
April 15th, 2013, 07:38
you swapped the dodge master? that thing is way too big for those brakes. stock (97+) MC/BB works great for the brakes you're running, my buddy had the same setup and it felt awesome.

Well, I had the 98'+ in mine (its a 2000 xj) and it felt way too soft. Wondering if the durango 1.125 bore would be what I need.

yossarian19
April 15th, 2013, 07:44
Yep. I think you need the 1.125 MC. Dana 44 brakes aren't 3/4 ton truck big.
Go with the Durango cylinder or Dana 60s.

Talyn
April 20th, 2013, 08:19
What are you running for a prop valve?

Tacedaddy
April 20th, 2013, 10:08
Im running a WJ booster and master for my 8.8/D30 setup, Works EXCELLENT

Talyn
April 20th, 2013, 17:41
That won't help him. The WJ master cylinder bore is the same size as his XJ's, 1". If there is any upgrade on the WJ system it is in the booster. However, I tried it and noticed no difference from my 2000 XJ setup. The WJ may be good for those with the single diaphragm booster though.

coloradoinsane
April 21st, 2013, 00:26
What are you running for a prop valve?

I ended up leaving it stock. I figured I would like to get the brakes to the point that I can at least lock up either the front or rear and work on fine tuning the front to rear proportion.

xjtrailrider
April 21st, 2013, 05:31
I'm going to ask the dumb question. Did you mount the rear calipers on the correct sides? The bleeder screw should be at the top.

I have the ZJ rear discs, stock 96 MC/booster and stock XJ prop valve with ceramic pads all the way around.

I can lock down anytime I need too. Sounds like to me you either have air in the system or you MC/booster has issues.

XJLI
April 21st, 2013, 06:32
I've driven an xj with your exact brake setup on 35s with a stock mc/bb and it locked 35 up no problem

rockthrasherxj
April 22nd, 2013, 17:50
i have a 92 xj with a 96 booster and a 99 durango master dana 44 rear drum added mid 80s dodge 1 ton wheel cylinders and can lock them up no prob on 35s

Ericlouie1
April 23rd, 2013, 13:02
If you didn't touch the proportioning valve, you should, when you get the system correct, you are going to need a reduction in pedal pressure to the rear, as they will lock up faster as the rear unloads, and the front takes more load under braking.
I've seen some systems where no matter what you do, you need to pressure bleed the brakes, as opposed to pumping th pedal, or using a vaccum pump. Case in point is my buddies zj, we ended up removing the abs block, and after repeated attempts at pump/hold, big vaccum bleeder, and pump/hold again, we ended up having to build a pressure bleeder out of a bug pump to get all the air out.
If the pedal is too firm, you MC choice is too large for your application, remember, you are using a 1/2 ton caliper in the front, and a car caliper in the back, so stuff designed for trucks may still have too much volume. I know from talking t ofull size guys that those eldorado calipers are less than ideal in heavy situations, just not enough surface area to make a vehicle stop good.
The other problems may be is you choice of pads and rotors.

cal
April 25th, 2013, 09:26
It sounds like you have air in the system.

I've got a 44/60 with chevy half ton front brakes and explorer rears, with a stock 95 master cylinder and booster. Pedal is rock solid and I can lock up 37's with Black Magic pads.

We're not talking massive brakes here, and our stock master cylinder is more than sufficient.

blistovmhz
October 7th, 2015, 10:57
Going to revive this one as well.

I've had a few threads on the subject of WJ fronts, KJ rears, with XJ MC and booster. Long short, the fronts lock up easily while the rears barely grab. There is no air in the system and the prop valve is not the problem (i've removed it to test, but also, I've verified that the prop valve is still open when the fronts are locked, while the rear's can still be turned by hand).

I haven't heard of anyone else with the WJ (akebono dual piston) fronts, with the KJ rears. I'm wondering if anyone can answer this:
In the XJ MC, is the secondary piston smaller than the primary (ie: front piston larger than rear, thus more fluid volume to front than rear), or does anyone know if the KJ rears are just way higher volume than the WJ fronts?

cal
October 7th, 2015, 10:58
I havent actually installed KJ rears, but I've looked at KJ fronts, and they are tiny compared to a WJ.

I would think the problem lies somewhere else.

blistovmhz
October 7th, 2015, 11:06
I havent actually installed KJ rears, but I've looked at KJ fronts, and they are tiny compared to a WJ.

I would think the problem lies somewhere else.

Somewhere else such as?
One note: If I bleed everything out real good, the pull a rear caliper, give the pedal a half a pump or so, then manually compress the rear piston back in and re-install, the rears work much better. Still crap, but they at least contribute to my braking somewhat. Still no where near being able to lock them up on even wet pavement.

I can't find any bulging lines (all steel and steel braided), and all 4 calipers are now brand new, as well as new pads/rotors. The other test I did was to have my buddy step on the brake as hard as he can. This locks the fronts, but the rears can still be turned by hand usually. While he's still on it, I cracked a bleeder on the front so the pedal would come down further, thus forcing more fluid to the rear, and sure enough, the rears locked up (at least to the point I coudln't turn them by hand, and they're capable of stopping my engine at idle.

The fronts work ridiculously well. On dry pavement in 2wd, they have no problem locking up my 35's. If I throw it in 4x4, the fronts will actually lock the rears through the driveline. This works fine on the trail, but on the street I can't be in 4x4, and ultimately on the street is where I really need the brakes to be working.

I've found fluid volumes, piston bores and strokes for every MC and caliper except the WJ and KJ calipers, and can't find anyone who's actually confirmed the bore/stroke of BOTH pistons on the XJ MC. Everyone says they're 50/50, but that I suspect is just a guess, as no one ever seems to post the actual bore/stroke.

I'll probably pull the MC tonight and tear it down to measure for myself, but even if I find that both MC pistons are equal stroke/bore, I still don't know the volumes for the calipers. For all I know, the KJ rear requires twice the volume of the WJ fronts. *shrugs*

cal
October 7th, 2015, 11:08
Without having to reread the entire pair of threads, what master and prop valve are you currently on?

blistovmhz
October 7th, 2015, 11:16
XJ master, and everyone keeps asking about the prop valve, but I don't understand how that's relevant as my problem starts LONG before the prop valve pops the rear closed.
But, just cause, I've tried both the stock XJ , as well as a ZJ 4w disc prop valve. Changing the PV had no affect because the fronts are already locked up and preventing the pedal from going further down, while the rear's can still be spun by hand.

Think of it this way. If you cap off your front output from the MC, but leave the rear connected, now hit the brake pedal. The pedal won't move because the front is already as far as it can go. If the pedal can't move far enough to push the secondary piston (for the rear), the rears aren't going to do anything.

I've also removed the prop valve entirely just to appease several other guys who were sure this was a PV issue. it's not. If I use my power seat with a block of wood wedged into the brake pedal, I push the pedal 1". At that point the fronts cannot be turned by hand and the rears can. If I push it to 2", the rears start tightening up, but can still turn by hand. The PV doesn't pop until 4".

In real world driving, 3" of pedal is enough to lock up the fronts on dry pavement. Again, tested with the PV removed entirely to the same effect.


Loads of guys run the WJ fronts with the XJ MC without issues, and loads of guys run the KJ rears with the XJ MC without issues, but I've never found anyone running both. My suspicion is that either the KJ's require way more fluid volume than the WJ's, or my MC is buggered and the secondary piston is somehow not moving far enough.


Actually, that gives me an idea. I recall losing a rear brake line last year to a tree, and I lost ALL brake pressure. I've never gutted an MC, but I'm assuming the primary piston is directly attached to the secondary, and that those two should be providing fluid isolation so that if you lose a brake line, you should still have pressure to the other end that didn't get ripped off.. Correct? If this is the case, the fact that I lost all pressure would suggest that the isolation between front and rear pistons in the MC has failed, in which case, that secondary piston is probably not really sealing well. Sound about right?

cal
October 7th, 2015, 11:18
I'm not blaming anything, just trying to get an idea where you're at.

Let me ask the question differently;

what year XJ or ZJ are the current master cylinder and prop valve from?

They change a lot more over the years than most people think.

blistovmhz
October 7th, 2015, 11:21
98 XJ MC and block. 98 ZJ PV. Also 98 XJ PV. Swapped PV's several times even though I can't imagine any way the PV has anything to do with it but everyone keeps telling me to check it again :). I strongly suspect most guys don't know how the PV works, and think it acts as a bias valve during all braking operation, rather than just bias during extreme pressure.
Also, this wasn't my thread to begin with. Just highjacked for visibility because i"m losing my mind. My original thread here: http://www.naxja.org/forum/showthread.php?t=1109833

cal
October 7th, 2015, 11:24
If you think volume is the issue, you throw a master from a dodge durango onto that 98 booster, and it will increase volume to both ends, but to the best of my understanding of the components you should not be having a volume or pressure problem. Make sure you properly understand how to adjust the peddle rod, which you may have to trim down a bit with a grinder.

Can you post 4 or 5 good quality pictures of your rear brakes, from a few angles?

blistovmhz
October 7th, 2015, 11:36
I can later tonight.

I suspect volume is the issue, but not volume in general. Just the ratio of volume between front and rear. Either the MC provides more volume to the front than the rear (stock XJ MC), or the WJ require less volume than the KJ. Either would make sense.

THrowing in a larger volume MC probably doesn't solve the problem because it'd just lock the front up earlier, and the rear (secondary) piston still wouldn't be able to travel far enough to provide that extra volume to the rear calipers, no?

Perhaps I'm just out to lunch on the MC internals, but from what I've seen from diagrams, pedal connects to the rod, which drives the primary piston, which drives the secondary, so the stroke of both pistons will be identical. If the bore of both MC pistons is the same, you'd have equal fluid volume to both outputs, but I feel like it'd make sense that the XJ MC, being setup for drum rear, would have a smaller bore for the secondary (as the drums require less volume during all braking). Is this not correct? Wouldn't a bigger MC just lock up the fronts a little earlier and have absolutely no affect on the volume actually going to the rear as it's limited by the volume that we can force to the front?

Wouldn't a bad seal in the MC secondary piston, or the secondary being smaller bore result in precisely what I'm seeing?

cal
October 7th, 2015, 11:40
I just cant believe that a KJ would require that much rear volume.

It just is not believable. But if it were the case, I would throw those brakes in the trash and get the same ford 8.8 brakes everyone else uses and be done with it. You've spent more time on this than the value of the parts are worth.

blistovmhz
October 7th, 2015, 11:43
That may be the right answer, but as I just bought new pads/rotors/calipers, I'd hate to go out and buy another set of 8.8 plates, calipers, pads, rotors, and shoes, and find out my MC is to blame. The fact that there seems to be no isolation between front and rear circuits makes me suspicious. I think I'll just crack that MC open and take a look, check for any failures, and give those bores a measure, as no one has ever posted about whether or not the bores are identical. They probably are, but I hate assumptions.

cal
October 7th, 2015, 11:47
I've installed a lot of brake upgrades on XJ's at this point.

The best by a long shot was prototype black magic kit. Dodge 1500 front stuff and ford explorer rear. 98 XJ booster, Dodge Durango master, 99 XJ prop valve.

http://i72.photobucket.com/albums/i186/naxja/431/IMG_20150530_161403858_zpsha8qdznq.jpg

http://i72.photobucket.com/albums/i186/naxja/431/IMG_20150530_161420163_zpswpboguj2.jpg

Never took a picture with the master installed.

http://i72.photobucket.com/albums/i186/naxja/431/IMG_20150530_171115252_zpslisdzude.jpg


You're really not far off of that keeping your WJ fronts. This had much bigger diameter rotors but not a lot bigger caliper.

http://i72.photobucket.com/albums/i186/naxja/431/IMG_20150530_110219647_zpsw7vnjtkc.jpg

But I just can't fathom the volume of the rear being a problem at all. Maybe I need to order up some KJ parts and compare them to my 8.8 stuff just to know.

blistovmhz
October 7th, 2015, 11:49
Well, I've got an old set of KJ calipers going back for core anyhow, and I already popped one of the pistons out. If you give me direction, I can go measure and post the numbers. I figure just knowing the bore should be enough right?

cal
October 7th, 2015, 13:02
Yeah, get me the bore and i'll compare it to the stuff I run.

cal
October 7th, 2015, 15:07
well it looks like the 8.8, ZJ, KJ and Crown vic all user the exact same bore rear caliper.

the problem is not volume.

yossarian19
October 7th, 2015, 15:48
Loads of guys run the WJ fronts with the XJ MC without issues, and loads of guys run the KJ rears with the XJ MC without issues, but I've never found anyone running both.


I ran WJ front and ZJ rear (same bore as KJ rears) with a 98 XJ master, with and without the ZJ prop guts. Braking was awesome. When I installed new rear pads, I noticed the difference - leading me to believe the rears were attending the party, though I never did try to spin them on a jack with the brakes engaged (why would I?)

Switched to Durango (Dakota?) 1 1/8" master cylinder. Pedal felt great but performance decreased for a given level of effort.

blistovmhz
October 7th, 2015, 18:02
I ran WJ front and ZJ rear (same bore as KJ rears) with a 98 XJ master, with and without the ZJ prop guts. Braking was awesome. When I installed new rear pads, I noticed the difference - leading me to believe the rears were attending the party, though I never did try to spin them on a jack with the brakes engaged (why would I?)

Switched to Durango (Dakota?) 1 1/8" master cylinder. Pedal felt great but performance decreased for a given level of effort.

That's the thing with the WJ calipers. They work so well that if you do both the WJ and the rear discs at the same time, you think they're both part of the massive upgrade. I did the KJ rears first and noticed a pretty big upgrade, but when I went to the WJ fronts as well, that's when I noticed the rear was weak. Like, might as well not bother with rear brakes weak.

I believe I have found evidence that the XJ (like almost all MC's) MC secondary bore is in fact slightly smaller than the primary (which makes sense). Someone in another thread (same topic) confirms that the KJ caliper does have a larger bore than the ZJ, which would pretty much confirm my theory.
I got lazy and it's raining out so I'll measure mine later or tomorrow to confirm.

raptor314
October 12th, 2015, 22:57
X2 on the front WJ brakes.

I swapped in an 8.8 and WJ booster and master, (as my ABS pump was making noise on my 90 XJ). With the XJ front brakes and the 8.8 rears it stopped ok, just barely locking up 35's. I had, (well actually still have) a pedal problem. I could lock up 35's, but the pedal would go to the floor if I kept pressing. I bled and bled and bled, tried 2 different MC's and boosters, replaced every inch of line, replaced the rear calipers and pads and same thing. Tried it with and without XJ and WJ proportioning valves and no change, front brakes would barely lock up and the pedal would go to the floor if I continued pushing.

I blocked off the front line at the MC and tested stopping with just the rears. The rears don't do much. The rears barely slow it down. But the pedal was rock hard with very little travel.

I did the WJ knuckle/brake swap and it locks up 35's much harder than it did with the XJ front brakes. (However the pedal will still go to the floor).

My conclusions, the WJ front brakes grab alot harder than the stock XJ brakes, and the rear 8.8 disks don't do much.

And I've got no idea why my pedal goes to the floor, but I can live with it because it stops so freakin' hard.

raptor314
October 12th, 2015, 22:58
Oops. Double posted.

Talyn
October 13th, 2015, 04:38
Tried it with and without XJ and WJ proportioning valves and no change, What WJ uses a prop valve? AFAIK they all use the ABS pump to modulate the brakes.

raptor314
October 14th, 2015, 08:29
What WJ uses a prop valve? AFAIK they all use the ABS pump to modulate the brakes.
Good catch on the PV. I meant ZJ. I'm currently running it now without the PV.