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CJ rear disc drag

1K views 10 replies 3 participants last post by  theytookmyname 
#1 ·
1983 CJ7

Dana 44 front with GM style calipers
AMC20 rear with 300ZX calipers


After swapping the rear discs in, I went to pull the residual valve for the rear from the MC, but it wasn't there. People I talked to thought I was looking at the side for the front brakes, so I triple checked with my Haynes/Chilton/Clymer manuals. Also confirmed because the same res. drained (and was filled) when I bled my rear brakes. I was told to drive it and see what happened. Well, the rears are dragging fairly badly. It's driveable, but it won't coast down a fairly steep hill out of gear.

Various people I have talked to said it had to be the stock CJ master cylinder, and not an issue with the stock combo/proportion valve. I swapped in a NEW master from a 1968 Corvette after extending the CJ brake rod 3/8" of an inch to work with the Corvette MC. Now, the CJ stops better than ever, but the rear brakes are STILL dragging.

So am I wrong in thinking that it's the combo valve? I have read that a ZJ disc/disc valve is a good alternative, but I haven't found any info on whether the lines/fittings are the same size as the stock CJ ones.

Thanks.

(I have searched, but there is a ton of conflicting info on this)
 
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#2 ·
Park it where it can roll (not steep, but enough angle to roll) and start cracking the rear line one place at a time starting at the master and working down the line to find out what will make it roll away (have someone to stop it so you don't get killded). Once you find the fitting that lets it roll, find out why that valve or fitting is causing it to hold pressure. If you get all the way to the calipers in the rear and it still won't roll, your calipers are the problem.
Travis..
 
#3 ·
With the rear line disconnected at the MC, the rear calipers still drag. I can't break the line after the combo valve because the fittings are seized.

if it's not the combo valve, is it possible that the hardlines are collapsed internally and are holding pressure? They have been twisted around and bent in and out of shape multiple times.

One of the calipers was seized completely but has been replaced with a reman.
 
#4 ·
If you crack the rear bleeders does it still drag? If so, caliper(s) are to blame. Try to crack the fittings at both ends of the hose that goes to the axle if the bleeders don't work, hose may be fubar internally. You need to keep jacking with it until you find something that makes a difference, or at least narrow it down to a something logical and replace the suspect part.
Travis..
 
#5 ·
If it has the stock proportioning valve then that is your problem. It's designed to hold a small amount of pressure to the rear brakes because they were drums.
On the rear brake output of the prop valve take out the piston/plunger and spring, turning the prop valve into a clear block and your problem will be solved.
Search "disc brake conversion" in jeep hardcore and there are some threads detailing with pictures what I'm talking about.
Good luck
 
#7 · (Edited)
Went to take apart the combo valve and remove the residual valve portion, and the rear brake line snapped off completely, leaving the fitting stuck in the combo valve. If that hadn't happened, I would have already put this POS together and taken it for a drive...

So what are my options now? I guess I'll have to pickup a ZJ valve to replace the rear portion with the fitting stuck in it, so I might as well swap the whole thing in, as long as the other lines aren't stuck.

Is there anywhere that sells the rear hardline only? I don't want to pay $2xx for a hardline kit when I only need one.

I guess I'll go pull a ZJ combo valve from the salvage yard next week.

EDIT: just looked at the ZJ combo valve design, and I have to swap the whole thing. Can't use the rearmost portion because it's not the same style valve as the CJ one.
 
#9 · (Edited)
Went to take apart the combo valve and remove the residual valve portion, and the rear brake line snapped off completely, leaving the fitting stuck in the combo valve. If that hadn't happened, I would have already put this POS together and taken it for a drive... Take your time and carefully remove the fitting

Is there anywhere that sells the rear hardline only? I don't want to pay $2xx for a hardline kit when I only need one.I'm sure there is. Try google. Or buy some hardline from your local auto parts store and bend it to match. Or just a new fitting flared onto your existing hardline would work assuming you didn't lose more than an inch or so of the existing line.


Or, I just took my stock hardline off my cj. You want it?
 
#10 ·
Thanks for the offer, I'll let you know if I'm interested. I haven't decided what to do now...

Since the Corvette MC is dual chambered, I don't really need a combo valve anyway. I'm considering getting an adjustable inline prop valve for the rear. I'd need a Tee fitting for the front line because the lines split coming out of the combo valve.

I'll stop by the local hot rod shop and see what they have when I get the chance. The rear line is 3/16" and the front is 5/16" right?
 
#11 ·
Change of plans.

Went to pull the stock combo valve and all the brake line fittings are frozen, even after liberal amounts of PB Blaster over a several day period. Looks like I'm sticking with the stock valve for now.

I should be able to use the rearmost portion from any GM combo valve from a similar year right? If so, I'll just hit up the local salvage yard for a replacement

How do I remove the inner valve portion? I tried to pull it with needlenose pliers, but no luck.

Thanks.
 
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