Brakes locking up -

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post #1 of 6 Old 05-01-2013, 04:27 PM Thread Starter
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1983 CJ7 
Join Date: Feb 2013
Posts: 115
Brakes locking up

83 cj7
Just got the jeep-79k miles. The guy I bought it from put around 1500 miles on it in about 7 years, so it has been sitting a good bit.
After about 15 minutes of driving the brakes lock up. Last time it happened we jacked it up and all
4 wheels were affected (disc front & drum rear)
When I first got it there was evidence of leaking rear wheel cylinders so both of them have been replaced with new shoes as well. The fact that it happens to all 4 wheels kind of rules out a crimp in the flexible brake lines.
I find I real peculiar that it only happens after driving for a little bit. Could it have something to do with engine heat having some sort of effect on the master cylinder?
Any help would be appreciated.

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post #2 of 6 Old 05-01-2013, 05:40 PM
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1984 CJ7 
Join Date: Nov 2008
Location: Hazel, KY
Posts: 1,250
something in your brake system is restricting the reverse flow of brake fluid.
if all 4 of your brakes are locking up, then as you said, I would rule out restricted or defective rubber hoses at first blush.

I would change out the master cylinder and start from there, if that does not fix the problem then rebuild all wheel cylinders and replace all rubber brake lines.

flush all old brake fluid,

at least that would be my plan, if these item replaced or repaired does not fix the problem, then it would be off to the mech shop

as you must have good dependable brakes to drive the jeep on the highway.

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post #3 of 6 Old 05-01-2013, 08:07 PM
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2004 WJ 
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Location: Stouchsburg, PA
Posts: 10,284
Master cylinder is the first to get replaced.
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post #4 of 6 Old 05-01-2013, 10:54 PM
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1982 CJ8 
Join Date: Oct 2012
Location: San Jose
Posts: 65
If your brakes are boosted check that the booster rod is releasing fully from the master cylinder. In mine I had to adjust the booster rod due to the MC never fully releasing pressure. During usage the brakes would build pressure until they lock up.

The spec was about 1/16" rod to MC plunger
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post #5 of 6 Old 05-02-2013, 03:08 AM
John Strenk
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1976 CJ5 
Join Date: Feb 2005
Location: Shalersville Ohio
Posts: 15,682
I had a truck with a bad booster that would lock up the brakes after a few min of driving.

Quite annoying because I would have to shut the engine off to get it to release the brakes.
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post #6 of 6 Old 05-02-2013, 06:38 AM
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1983 CJ7 
Join Date: Jul 2010
Location: North Andover, MA.
Posts: 211
FWIW, mine always does this in the spring, after being stored all winter. I always attributed it to rusty drums (rears lock, not fronts). It stops after a couple of days driving....

However, this is not to say you have an issue.....

Just my .02
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