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2000 pedal sinks AFTER new MC, hoses, calipers, pads

92DripCherokee

NAXJA Forum User
Location
Tampa, Florida
My 2000 classic brake pedal sinks. It still does same exact thing AFTER a new MC, hoses, calipers, pads, and 3 brake bleed/flush cycles.

At one push the pedal is high and firm, and gets lower and spongier with each push. Every once in awhile it drops slightly, and fairly suddenly, while sitting at a stop. Give it a second to rest, and the pedal feel returns.

This isn't like brake fade, it even happens simply when pumping the brake at a stop, and while driving at all speeds. Unlike a loss of vacuum assist, IT GETS SOFTER!

The booster passes airtight tests, and while the engine is pulling vacuum, the pedal gets softer with rapid pedal pumps. The booster has vacuum reserve left after sitting overnight.

There are no fluid leaks, or fluid loss, the wheel cylinders are ok. No leakeage at the calipers, or the hard brake lines. The brake hoses are spanking new.

Is this "sinking feeling" a sign of impending doom? The stupid brake techs who drive it briefly never remark on it! My best guess (without the DRB III tool) is an internal ABS leak. Anyone have an idea? No ABS light has ever lit up either.
 
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Can't suggest much...seems like you covered all the bases.

Have you tried disabling the ABS...pulling the fuse? You might give that a try...just make sure the road is nice and dry.

Also, how did you bleed your brakes? With ABS, I believe there is a specific method or tools needed to bleed them properly...not sure if the old 'pump 3 times and hold' method is still valid.
 
The brakes were vacuum bled. They respond firmly on the initial push, and then get mushy, then recover. The MC is a brand new unit from Carquest.

After doing much internet digging, the symptoms sound like a "bypassing" master cylinder (leaky cup seals). But I doubt the new MC would behave like the old one.

Im still try to figure out if my 00 really has RWAL (rear wheel antilock) brakes. I can't find squat on diagnosing this system. Not even in the FSM. So unless someone here recognizes this one, I'm going to wind up in the evil clutches of the stealership. It definately doesn't have 4 wheel ABS.

http://jeepsunlimited.com/forums/showthread.php?t=613054

this JU thread sums it up exactly, but I'm no closer. No one knows squat about RWAL! And the FSM only mentions it in passing.
 
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Uh, that doesn't sound quite right...if you have ABS upfront, you should have ABS out back...some vehicles were equiped with rear only ABS, but never heard of a vehicle with ABS just up front. (If it did, I'd really, really be surprised with Chrysler for making such a cheap system.) Makes no sense as the rear is more likely to lock up.

Now, maybe the XJ didn't have the more expensive 4-channel ABS...sensor at each wheel, not sure. But maybe it has the less expensive 3-channel ABS system...one sensor at each front wheel and one for both wheels in the rear.

I'd try bypassing the abs system and see what that does...that will give you the more traditional braking system.

good luck.

P.S. So, you did the bench bleed on the new MC before installing? Are the rear brakes adjusted properly...just barely dragging against the drums? Also, I'm not sure the vaccum method of bleeding works too well with ABS systems...I was told that you needed to use the positive pressure method...force fluid via the reservior.
 
I'll try pulling out the ABS relay from the PDC, and give it a whirl. Something weird is under the hood of this thing. I do have ABS plumbing under the proportioning valve, but theres no front ABS sensors or wiring running to the front wheels.

That leads me to believe it only has rear wheel ABS, or worse, it was f----d with.

It was bench bled on a vise. There still may be air little in the ABS solenoids system, but how much will a dealership charge me to hook up a a DRB-III scan tool and bleed this thing?
 
Hmmm...sounds like it was f---d with.

I don't think you have to take it to the dealer...any good brake shop should have necessary tools to properly bleed/set an ABS system.

Maybe someone here can chime in about XJs having just 2wheel ABS???

Oh, one more thing...when you mentioned that your pedal gets softer...that's while the engine is running, right?

How does the pedal feel/react when you repeatly press it when the engine is not running? It should not go to the floor with no vaccum assist.
 
The hydraulic pressure is letting go somewhere, it has to be some kind of internal leak.
the pedal never goes to the floor, even with the engine off. Its travel halves after the second push, and is solid by 4 pumps. This is normal vacuum booster operation.

And the ABS system, according to the FSM, has to be controlled by a scan tool for proper bleeding. I've heard of shroting some relays to start the ABS pump during manual bleeding, but its too risky.

Thanks for your input, I'm going to look around the jeep some more, and determine what to do next.
 
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I used to have ABS on my 98. If you look on your ABS manifold it should have 5 hard lines attached to it. Two come from the Master Cylinder (Front and Rear) and the remaining three go to the brakes themselves. Two are for the Front (Left and Right) and the third controls both rear drums.
 
92DripCherokee said:
Ok, there's no separate ABS circuit. XJ's either have 4 wheel ABS or non-abs brakes.

That was easy.

They have 4 sensors and 3 brake lines. The rear wheels each have a sensor but a common fluid source. The front are independent. I guess you can call it a 3.5 wheel ABS system :)
 
Try the pump and hold bleed method yourself. Sounds like there may still be air in the system. I just did a 4-wheel brake job myself, and I am no mechanic. I replaced the front caliper hoses and the single rear axle flex hose. I did the bleeding / flush myself with someone pumping the pedal nice and slowly. I have four wheel abs on my 01 xj, but I didn't change my MC like you did. My mc is original. I did replace my rear wheel cylinders with new ones, not rebuilts. Drove my Jeep to work for the 1st time in 6 months and it stops great.
 
It's very possible some remaining air is trapped in the system. But over the past couple of days the brake pedal has become firmer! That might sound ridiculous. Figuring out I did'nt have ANY form of ABS brakes, now that was ridiculous.

I have a theory, the seals in the new MC swelled and "fluffed" out from soaking in brake fluid and getting more pliable from engine heat day after day. It "broke in". Maybe the cup seals were a bit shrunken from being old shelf stock. Maybe it was "bypassing", but it tightened up with use!

The MC was bench bled, but I'm sure some air got into the lines right before the fittings were threaded in. I really want a reverse bleeder gun. That would get the bubbles out of a 747's brakes!

But I really don't know if the brake system rubber parts would actually absorb the glycol dot-3 fluid. Its not like a petroleum based system, where certain rubber compounds react with certain additives.
 
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I had a similar problem when I did the rear disk brake conversion. To fix it I ended up having to flush the whole brake system with 2 big jugs of brake fluid. There was ton of tiny air bubbles and crud that kept coming out. First I put a rag under the master cylinder and bled it by cracking the lines. Then I gravity bled the calipers with a hose and jar starting at the closest wheel (drivers side). By the second jug the crud and bubbles stopped. My wife was totally relived when I told her we were done. :twak:
 
So maybe crud was fouling up the seals in the MC, that is possible. The brake fluid is very clear and clean, but it could have been a fragment of rubber too...hard to tell.
 
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