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  #1  
Old October 7th, 2020, 18:20
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Dzldust Dzldust is offline
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Rear brakes lock up

1996 XJ
Rear brakes lock up on moderate to heavy braking
Not a mud Jeep or water trails ,no brake fluid leaks or axle seal leaks
Swapped out front rotors,calipers and pads -no change
Swapped out rear drums,cylinders and shoes - no change
New soft brake lines ,bled entire system-no change
Only thing left is Master cylinder and prop valve,non-abs XJ
Good firm pedal and rear brakes are adjusted on the money
Emergency brake cable in good adjustment as well
Could this be a simple proportioning valve problem? Jeep has been a street Jeep and not abused or hacked on
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  #2  
Old October 8th, 2020, 05:37
Unclewolverine Unclewolverine is offline
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Re: Rear brakes lock up

Well, years ago when drum brakes were the norm i read in a fsm, not sure what make or model, that the brakes were properly adjusted when under hard breaking the rear brakes lock up and the fronts don't. I know my non abs 01 would lock up all the time delivering mail but that was gravel roads, not on pavement, which I assume is what you are talking about.
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Old October 8th, 2020, 05:40
SlowXj SlowXj is offline
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Rear brakes lock up

Weirdly my 96 would do this during rain, moist climate conditions. On a paved parking lot at 5mph I could apply the brake pedal to slow down (normal deceleration) and the rears would randomly lock up.... It continued to do this for 10 years and many pad,shoe,drum, and rotor changes... The disk brake 8.8 fixed all those issues.

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Old October 8th, 2020, 07:10
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Re: Rear brakes lock up

Maybe I’m just used to all my newer vehicles that just work ,honestly this may be normal for the era and I’m just trying to fix a problem that doesn’t exist
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Old October 8th, 2020, 07:11
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Re: Rear brakes lock up

I pulled the proportioning valve apart and it looks clean but the front shaft behind the bleeder had some gunk on it ,cleaned it up and brakes still lock up rather easy,maybe master cylinder ?
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Old October 8th, 2020, 10:11
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Re: Rear brakes lock up

Cheap drum brake shoes absorb moisture and then get grabby. I had a Bronco II that would lock the rear brakes two times every morning, once at the end of the driveway, and once at the end of the block. Then they were fine until the next morning.

If you are going to shotgun random parts at an undiagnosed problem, I would start with something inexpensive like better quality brake shoes.
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Old October 8th, 2020, 10:18
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Re: Rear brakes lock up

Not throwing random cheap parts at it,not sure what made you think that ,havenít been in rain or water ,thanks for your input
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Old October 8th, 2020, 11:32
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Re: Rear brakes lock up

Alright just changed out the Master cylinder With a new one,not reman,re-bled all the brakes and took proportioning valve apart again and it all looks clean. The Jeep does have a little forward rake to it,front is prob an inch or so lower than the front,I’m starting to wonder if the rear end is unloading and getting light causing the rears to lock up easier than the front on fairly hard stops.Jeep hasn’t seen moisture ,rain or any other source of water on brakes . Again no oil leaks and everything is quality and brand new over the last few days .
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Old October 8th, 2020, 11:55
RCP Phx RCP Phx is offline
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Re: Rear brakes lock up

If think that is a possible cause, level it out and/or get some good shocks.
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Old October 8th, 2020, 12:34
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Re: Rear brakes lock up

It’s the only thing I can figure ,starting thinking outside the box ,if you replace everything related then it has to be something else , When I got the Jeep it had a welded rear diff and old BFG km2’s, figured I had a Wheel cylinder locking up and with the diff welded it would just lock both rear brakes , well after changing out the carrier and a new set of tires It kinda ruled out the welded diff and old rock hard tires as the culprits, that when the brake trouble shooting started .Once I took the other things out of the equation . Now I’m down to either the rear end getting light or a bad proportioning valve which I have on order.ill check back once it’s swapped out
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Old October 8th, 2020, 18:35
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Re: Rear brakes lock up

How high is your XJ lifted, how much down travel remains on the rear shocks? Rear unloading is a possibility.

When my XJ did that frequently it was due to:

1) it was lifted 3.5 inches and still using the OEM length shocks in the back.
2) with drums in the rear, a normal humid South Florida weather would cause
the rear to lockup on the first couple of break applications. I countered this
by driving with the breaks lightly applied 1/4 mile or so. It was worst after
an over night rain.
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Old October 8th, 2020, 19:10
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Re: Rear brakes lock up

Shocks are proper length if anything too short of a shock would limit unloading once they maxed out like a limiting strap
Itís been dry here plus I drove the Jeep minutes after putting rear shoes,drums and new cylinders in and it locked up,no chance for moisture and itís been consistent no matter what I do. I had a long armed TJ that would do the same thing years ago,I used Ome coils and the rears were heavy duty rate for the tire carrier.It sat about 2Ē higher than the rear,It would also lock up under hard braking ,never thought about it until now but once I leveled it out it never did it again as far as rear brakes locking up . I have Acos spacers up front,guess Iím gonna see if I can level the XJ out and see if it changes anything,gotta find my scanner wrench though
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Old October 8th, 2020, 19:21
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Re: Rear brakes lock up

I see what your saying now that I thought about the shocks not having enough down travel
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Old October 8th, 2020, 19:21
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Re: Rear brakes lock up

It’s lifted 4.5”
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  #15  
Old October 9th, 2020, 13:02
2X_01_Jeeps 2X_01_Jeeps is offline
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Re: Rear brakes lock up

I too have experienced the first-stop-damp-lockup with my '84 Cherokee when I had it.



Just something to consider is that if the rear shoes were put in backwards you'd get excessive braking force. I think it goes that the shoes with the shorter length of lining (if you have that type) go on the front so that the servo force that helps "lock" the shoes when braking when going forward isn't so strong.


My Dad found this situation on a used pickup truck he bought for my Aunt.
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