86 xj, brakes will not bleed. Not a complete n00b.

[blistovmhz]

Before everyone jumps to "have you tried *obvious answer here*", yes, I have.
A buddy picked up an 86 XJ with an old carbed chev 4.3. We've got everything else working pretty tip top, but brakes are ****ing eluding me.

He had another of his buddies bleed the system. They found lots of air but by the time they were done, there was absolutely no affect on braking.
I take a little more methodical approach.

Original situation:
Pedal stroke at 2" provides enough pressure to stop the front wheels from being spun by hand, but rears haven't even moved. Verified by pulling drums. The Cylinders have not moved a nanometer with 2" stroke. At 3" stroke, the fronts are about as stiff as they're going to get, and rears still haven't moved. At just shy of 4" stroke, the pedal is rock hard and the rears start expanding slightly, but not enough to stop me from spinning them by hand. At exactly 4" stroke, the prop valve pops and of course the rears cannot possibly go any further.

So I bled the entire system (in the correct order, through about 5 full reservoirs of juice). Found that the fronts and rear driver all bleed normally, but rear passenger always ends up with air in it. Suspected cylinder was drawing air on retraction, and replaced it. No longer draws air, bleeds fine, but ZERO affect on pedal stroke.
Pull the MC and bench bleed it, reinstall, re-bleed entire system, zero affect.
Install new MC after a very serious bench bleed, re-bleed system, zero affect.
Bleed the combo valve (I'm not sure if it's bleeding properly due to it's location, but no more air coming out). Zero affect.
Replace some rear lines for no reason. Zero affect.

With engine off, first pump gets about 2" stroke. Second about 1", and third the pedal barely moves 1/4". With engine on and booster helping, pedal feels like it's not even connected to anything until it hits 2-3" stroke, then ramps up quickly to 4". Again, just before the prop pops, the rears haven't moved at all, but front pressure is as high as it's going to get. Above 4" the prop pops and pedal gets super stiff immediately.

Logically, we've got air in the rear circuit. That explains why the prop is popping before the rear cylinders have any pressure. Like, at 2" stroke, I can pull the drums off by hand easily. At 3" stroke, I can force them off.
Yes, the rear shoes are adjusted properly. We've even tried adjusting them so they're too tight to turn by hand before applying brakes. Zero change in pedal travel or overall braking performance.
Sorta at my wits end here. I always get the most ridiculous problems, but I'm the guy everyone comes to when they can't figure something out, and this one's got me stumped. We've got a buddy bringing over a pressure bleeder as a last resort. I suspect it'll work, but even if it does, I don't like not knowing why a brake system wouldn't bleed properly.

Am I missing something and just derping out? I'm not about to discount the possibility that' i"ve just suffered a stroke and am not thinking clearly.

 

[Jeep Driver]

Bad rear soft line or bad prop valve.

That's my guess.

 

[blistovmhz]

Both confirmed good, but I came up with a theory.

The MC that's in there is identical to the brand new one we picked up. According to the Internets though, the stock MC and 100% of aftermarkets are NOT the same. I'd have known this if the new MC came with all the parts it's supposed to come with, but it was missing the pushrod, which when I looked at another new MC, it's a completely different pushrod for a completely different master and booster. The pushrod in the truck matches the booster in the truck.

My theory here now, is that the old pushrod is too long for the newer MCs, so when you bolt the MC in, the piston is pushing the MC piston in like half way or something. Thus, bleed it all you want, but you'll only have half your stroke. Somehow the booster is eating this extra stroke up so you can't feel it at the pedal, but it does explain the fact that we're getting no volume to the rear, while the fronts are tight, but still can't lock up due to insufficient pressure/volume when the pedal hits the floor.

Does that make sense to anyone? We're thinking **** it, assume this is the case, and just upgrade to a 96+ booster/master so we know what we're starting with, and get the dual diaphragm to boot.

 

[blistovmhz]

Eyup. That was it. Maybe this'll help someone in the future.
For reference, this was an 86 cherokee with what we thought were stock brake components. Turned out the PO had replaced the MC with the newer aftermarket revision MC, but did not install the matching new revision booster. They're completely different units and do not work together.
With the OEM booster installed and using the OEM pushrods, the pushrod pushes the master piston in almost half way once it's bolted up, so when you bleed, you're only bleeding for the last half of your stroke. You'd think it would be obvious because your pedal would only have half stroke, but it's a two piece pushrod, so the pedal side stays with the pedal and the MC side stays in the MC, while the booster holds the secondary rod at half stroke in the MC. It FEELS like normal pedal movement, but it's just the booster ****ing with ya.

We ended up just swapping one of my old 98 XJ MCs in, with a new 98XJ booster (dual diaphragm), modified the pushrod (one piece) to fit the 86 pedal and brake switch, and everything works perfect now after a good bench (but not bench, and we just did it attached to the truck) bleed.

 

[Ecomike]

Interesting, everything you went through I have been through a dozen times over 4 years, and have yet to find a final fix for it. Mine is an 87 Wagoneer Ltd Ed. I too had a bad rear wheel cylinder sucking air on the return stroke and the replacement just pissed all over the driveway LOL. But I never had any problem bleeding the rears or getting them to lock up quickly as do the front ones. What I never solved was the 2-3 inch mushy pedal with the engine running. With no vacuum the pedal is rock hard in about 1/2" of stroke. I finally just put it back into DD service as the brakes work, they just feel like mush and have an extra 1-2 inches of mush before they can lock up the wheels.

 

[blistovmhz]

Interesting, everything you went through I have been through a dozen times over 4 years, and have yet to find a final fix for it. Mine is an 87 Wagoneer Ltd Ed. I too had a bad rear wheel cylinder sucking air on the return stroke and the replacement just pissed all over the driveway LOL. But I never had any problem bleeding the rears or getting them to lock up quickly as do the front ones. What I never solved was the 2-3 inch mushy pedal with the engine running. With no vacuum the pedal is rock hard in about 1/2" of stroke. I finally just put it back into DD service as the brakes work, they just feel like mush and have an extra 1-2 inches of mush before they can lock up the wheels.

Could very well be the same issue. We went out on a trip today (and omg the hilarity. EVERYONE broke except me, multiple times), and on the way down a mountain, buddies front brakes started to stink real good. He stopped (barely) and shut off his truck (which he shouldn't have done as we'd just turned around to go home because his starter wasn't grabbing the flywheel), and his fronts were scorchin' hot as expected, but rears were dead cold. They were locking up on wet pavement in the driveway before we left. I suspect the rears are unrelated and we just fixed the problem with the wrong MC/Booster combo.

Take a look at the OEM MC/Booster vs all "OEM" aftermarket replacements. If your brake pedal pushrod is curved up slightly at the pedal, that'd be the original OEM pushrod. What are the odds you're still rockin' a factory MC, 30 years and change later?

 

[90xj06]

when did they revise the mc/booster?

 

[Ecomike]

Take a look at the OEM MC/Booster vs all "OEM" aftermarket replacements. If your brake pedal pushrod is curved up slightly at the pedal, that'd be the original OEM pushrod. What are the odds you're still rockin' a factory MC, 30 years and change later?

Perhaps we can talk you into giving us more details, even photos if possible, and links? Maybe a thread on the topic????


This is the first time I or any one else here, I think have heard of this.

I do know that the vac booster on my 85 ( 4 banger, 2.5 L) is different from the 87 vac booster ( 6 banger, 4.0 L). I also know that there is a threaded adjustment on the rod that goes between the MC and the VB. I had to adjust it on my 87 when I replaced the VB.

But I have always wondered if there was some way the VB could be causing my problem, but until now no has seen or know of any way it could be the issue. And I have a hard pedal with the engine off which to me says it must be an air bubble that is forever trapped in my lines. I tried 5 new MCs already, and replaced everything else and bleed everything else 50 times over a 4 year period. Like you I am no noob to this. But you have given me a possible clue. Would love more details!!!

 

[blistovmhz]

Perhaps we can talk you into giving us more details, even photos if possible, and links? Maybe a thread on the topic????


This is the first time I or any one else here, I think have heard of this.

I do know that the vac booster on my 85 ( 4 banger, 2.5 L) is different from the 87 vac booster ( 6 banger, 4.0 L). I also know that there is a threaded adjustment on the rod that goes between the MC and the VB. I had to adjust it on my 87 when I replaced the VB.

But I have always wondered if there was some way the VB could be causing my problem, but until now no has seen or know of any way it could be the issue. And I have a hard pedal with the engine off which to me says it must be an air bubble that is forever trapped in my lines. I tried 5 new MCs already, and replaced everything else and bleed everything else 50 times over a 4 year period. Like you I am no noob to this. But you have given me a possible clue. Would love more details!!!

I didn't delve too far into it because ... why bother when I could just upgrade to the 98 setup.

Here's what I do know.
The 86 era XJ (as well as a few other Jeep products in the same era) used an enormous single piece cast MC with integrated cast reservoir. They came mated to a single diaphragm booster with a two piece pushrod assembly.
The rear (pedal side) rod starts with the pedal eyelet, which has the rod attached sorta offset to the bottom of the eyelet. The end of the first rod lands in the booster and looks like a piston, about 1" diameter. The front end of the pushrod starts in the booster with a similar piston (1" diameter) and the end is small diameter and adjustable.
I'm not absolutely positive on this, but it looks like something inside the booster prevents either end of the pushrod from moving past the diaphragm. The two pushrods float in the booster and are in no way connected to each other.

I don't recall what I was searching to find this, but I found several posts here and on pirate, loosely describing this problem, and 2 or 3 people chimed in stating that the OEM factory MC was no longer available after sometime in the early 90's, and that a revised version was released with a single piece pushrod, which of course would not work with the factory booster as the single piece is just a normal rod with a consistent OD throughout.
When we picked up our new MC (the one everyone had listed for the 86), it was missing the pushrod and bleeder hoses, and I didn't even consider that anything was missing so we just bolted it in. The new MC looked exactly like the one that was in our truck, but we do not know if that was the factory MC or not. The casting was slightly different at the booster end, but everything else looked identical.

What I do know, is that we also asked for a new 86 booster just to look at it. It had the adjustable pushrod inside (which you'd think was strange as the master that's supposed to match it, has a completely different pushrod included as well). Overall, the pushrod lengths are very different.
So the store has a MC that definitely doesn't match the booster. Seems to me the MC is the newer revised version with the single piece rod, but they somehow have the original booster.
If you look up pictures of the factory pushrods, you'll find the two piece fairly often, but if you go to any parts vendor, they will have the single piece.

This is the revised version we found at the store (same pushrod from all manufacturers):

This is a brand new, not remanufactuered unit.

This is the adjustable pushrod that was in our truck, and it's the same one that comes with a brand new booster listed for this model/year (again, from several manufacturers):

We don't know if this is new or reman. I strongly suspect that a lot of the boosters are reman while no one bothers remaning the factory MCs.


This whole thing was too damned goofy to just give up here. I'll try to track down the few references I found to the MC/Booster revision as it really needs to be better documented. I'm willing to bet this is why EVERY one of my old pre-90 XJ's always had brand new MC/booster combo installed.
I'll also see if I can deduce anything from part numbers. There's gotta be a manufacturer who documented the revision at some point.

*edit* I did just check with rockauto and a few other sources. Most of the boosters ship with the two piece push rod, while most of the new MCs ship with the single piece. There are a few reman MCs, and they all ship without any pushrod. Rockauto has a "pedal rod code" listed for several boosters. Tres interesting right?

*edit 2* Called my parts shop again to take a closer look at their part numbers and pull a few booster/MCs off the shelf for comparison. They're as confused as I am. They cannot find a single MC listed for 86 that will fit the booster listed for the same year. They confirmed that all the new MCs they have, come with the single piece pushrod, while every booster they list for that year, also comes with it's own 2 piece unit, and also confirmed that the 1 piece cannot work in the 2 piece booster, and the 2 piece pushrod engages the MC on installation, by about 1" stroke, so they just can't work together.
Also called Omix-Ada. They were also confused. One of their guys suggested as I'd found, that the MC/Booster were revised, but they can't find a booster that fits the newer MC either.

I say **** it. If you end up with this problem, do yourself a favour and just upgrade to a 98+ MC/Booster. Gets you a cheaper Master with a separate plastic reservoir, as well as dual diaphragm booster.

 

[Ecomike]

Thanks very much. I have a few thoughts, right or wrong.

Sometimes the aftermarket folks use one part number and box of parts as a universal for many applications. Not every part is used in every application. I suspect the one push rod has no use and nothing to with our jeeps, especially if it does not fit. Perhaps the MC itself fits are vehicles just fine, and other GM rigs perhaps since much of the AMC jeep architecture was made of GM parts...

The engine sensors, electrical connectors, (alternators?) and steering columns were GM.

I have an 85, three 87s and an 89 XJ. I have replaced the Boosters, MCs, and on one, the brake pedal side rod (the 89) since about 2004.

In that time I discovered the 85 (2.5 L) has a slightly different sized vac booster, but the MC seems to be the same as the 87-89, 40 engine XJs.

I have only had two problems in all that time with replacing the MC and Booster one at a time. One was the wrong rod on the 89 that was causing a loose, iffy operating brake switch. I got a new brake pedal side MC rod (I forget how, new Vac booster?) for an 89 and fixed that problem. I suspect someone had pulled an HO brake set of parts and installed a miss match before we bought it.

The other is the 87 Wagoneer, that has been hounding me for 4 years now. Perhaps I got a rod pair with the new booster 4 years ago and did not swap it thinking I did not need too? IIRC I bought the booster by itself and the MC by itself, to insure I got the new cast iron MC, and not a POS aluminum rebuilt VacBooster with the rebuilt VB. I have been told for years the Vac Boosters are NLA new, only rebuilt.