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post #1 of (permalink) Old 04-29-2007, 09:58 AM Thread Starter
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CJ front disc problems-

I'm having problems with front brake locking up after a short drive on my 84 project jeep. New calipers, pads, master cylinder and booster. Even have new brakes, hardware and shoes on the back. New brake lines- hard and flexible. Replaced proportioning valve last weekend. I thought I had everything resolved...
After brakes cools down, jeep can roll. Seems like the problems start all over again...
HELP!!!! Any suggestions?
Thanks
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post #2 of (permalink) Old 04-29-2007, 10:08 AM
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For whatever reason it sounds like that caliper is dragging . Remans are not always perfect ...
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post #3 of (permalink) Old 04-29-2007, 10:13 AM
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Are all the parts stock for the year, or did you do an upgrade too? Can you pry the pads back so that there's noticable room between them and the rotor? When they get tight is it both sides or just one? When they get tight, quickly jack up the front end and see if the sides have tightened up equally. Also see if they release gradually or all at once.

There were two different series of front brakes on those CJs. If you got a mix of parts the pads might be too thick for the calipers and rotors.

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post #4 of (permalink) Old 04-29-2007, 12:30 PM Thread Starter
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Factory reman. calipers were replaced by units with new hardware kit by a professional mechanic. Front flexible lines replaced with braided steel units. Piping from master cylinder to proportioning valve correct.
When brakes lock-up next, I'm going to loosen up lines between master cylinder to proportioning valve and between proportioning valve to calipers. If there is pressure build-up, the pressurized fluid should help tell me where the problem is isolated at.
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post #5 of (permalink) Old 04-29-2007, 01:03 PM
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Unless the proportioning valve is completely garfed, loosening the rear line at the MC should release all pressure in the front circuit.

I've never seen it, but an incompatibility between the MC and the linkage and pushrod could theoretically cause that. If the MC isn't allowed to return all the way it will hold pressure in both circuits. A temperature increase could then apply pressure to the brakes, and the fronts will be a lot more sensitive to that. Just reach up under the dash and see if there's a little freeplay in the pushrod.

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post #6 of (permalink) Old 04-29-2007, 04:03 PM Thread Starter
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Quote:
Originally Posted by Jim_Lou View Post
Unless the proportioning valve is completely garfed, loosening the rear line at the MC should release all pressure in the front circuit.
After a short run, brakes started getting tight. I did loosen up rear line on MC and slight pressure WAS released- not much.
I guessing that non-power brake pushrod is the same as on a power brake rod. I don't believe I every had a non-power jeep to get rods mixed up. There was some freeplay under dash, not much.
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post #7 of (permalink) Old 04-29-2007, 04:03 PM
 
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1. Collapsed (Soft) brake line.
Discount auto parts companies can't be trusted for soft lines.
I've had more than my share of soft lines where the internal liner would leak and swell up, but show little or no signs on the outside.
To test for this, get the offending brake to 'Stick', then pop the bleeder open and see if that wheel is holding pressure.

2. Caliper in a bind.
Once applied, it's binding and is difficult to get it to disengauge.
This would be an alignment problem with bracket or caliper, bolts, ect.
Lots of people don't grease the rails, pins and bolts the caliper rides on when they do a brake job, and binding is very possible.

3. Piston not retracting after application.
This would be the piston in the caliper. Metal shavings or rust from old lines can get inbetween the piston and it's bore.
When changing calipers, you MUST flush the lines before you hook up the lines.

4. The piston or bore can also be some stupid shape.
You never know what some remanufacturer is going to try and do to save 3.
Always a good idea to test the pistons with compressed air before you install them.

5. Wrong master cylinder OR BACK PRESSURE VALVES for the application.
Guys changing from drum brakes to disc brakes often forget to replace the back pressure valves that keep the shoes/pads from totally retracting every time you let up on the pedal.
Usually build into the bottom of the master cylinder, but not always...
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post #8 of (permalink) Old 04-29-2007, 07:01 PM
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So when the pressure was released were the brakes free again?

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post #9 of (permalink) Old 04-29-2007, 07:33 PM
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Quote:
Originally Posted by Junk Yard Genius View Post
5. Wrong master cylinder OR BACK PRESSURE VALVES for the application.
Guys changing from drum brakes to disc brakes often forget to replace the back pressure valves that keep the shoes/pads from totally retracting every time you let up on the pedal.
Would be my guess too... Also known as "resiual pressure valves" = designed to keep a slight pressure on the drums (not enough to drag, just enough to react to quick pedal depression). Easily removed from a drum-drum master by pulling the flare seat (sheetmetal screw just enough to grip, but not deform the seat... yank with pliers), then the RP valve (same sheetmetal screw), and place the seat back inside.

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post #10 of (permalink) Old 04-30-2007, 06:31 AM Thread Starter
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Quote:
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So when the pressure was released were the brakes free again?
Yes, some. Brand new master cylinder and booster was bought for 1982 and newer CJ with front disk and rear drums. I'll double check if they gave me the correct package. Thanks!
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