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Explanation of Rear Brake Lines


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Well, I finally got around to my brakes again last night. I was able to free up all of the rusted connections using a locking pliers. :brows: Not a proper wrench, but not a sawzall either. Tonight I will be able to hook up my line and bleed the brakes finally. :D

While I was under the MJ, I traced the brake line around and had a couple of questions about the function of the junction boxes and such. I haven't had to work on this aspect of the truck to date so this is new to me.

So, here's my take on what's going on under the bed.

There are two brake lines from the master cylinder that run to the rear of the truck. One line goes to a junction box that is connected to the proportioning valve that operates off the axle (this is the line that ruptured). This box has a hard line that goes to another junction box by the bed rail that the other brake line from the master cylinder goes to. One line leaves this box and connects to the rubber line which is connected to a junction box on the rear axle (below the proportioning valve). Out of this box, a line heads to each wheel cylinder. Am I getting this right? Why two brakes lines heading to one line then to each wheel cylinder? Is this incase one line ruptures you still have some rear brakes?

Also, where does the rubber line from the axle go to (I couldn't see where it terminates) and what does it do?

Last question for now, when people swap axles what changes do you make to the brake system?

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Your description probably makes sense to you, but I know how the system works and it confused the hell out of me.

 

Here's how it works: There are two lines from the front to the back because one feeds the rear brakes through the height-sensing rear proportioning valve, and the other is a by-pass circuit that is supposed to deliver full (unproportioned) braking power to the rear in the event the front brakes lose pressure.

 

Okay, so which is what?

 

Start at the front junction box (combination valve), directly under the master cylinder. The two ports on top are inlets. The outlet ports at the rear of the brass block feed the front brakes. The port at the "nose" of the combo valve is the normal circuit feeding the rear brakes. The line from that is plumbed to the height-sensing proportioning valve. The forward outlet on the bottom of the combo valve is the bypass circuit. Nothing should flow in this circuit unless the front brakes lose pressure, in which case the slider that actuates the brake system warning light also opens a port to allow fluid through the bypass circuit.

 

The two lines at the rear come together again because both circuits need to feed into the brake lines on the axle. One is proportioned output from the height-sensing valve, the other is unproportioned output from the bypass circuit.

 

Having blown up a rear height-sensing valve in a panic stop, I no longer trust them. You have to do what you're comfortable with. I'm old enough that I grew up with vehicles that never heard of brake proportioing valves, so my preference is "keep it simple, stupid." That valve is one more thing to fail. I can live without it, very happily.

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Thanks for the reply, now I know what is going on under the truck. My explanation was not the best because I didn't fully understand what was going on for the rear brakes. This is my first Jeep and my first vehicle that needed brake work beyond bleeding and replacing pads (it's hard to believe and I have owned some real turds). I agree with you, keep it simple if you can. That is why I was confused for so long about the lines on the truck. I have lots of experience on motorcycles: master cylinder => brake line => caliper. Simple.

Also, last night I was able to replace the brake line and bleed all of the brakes. No leaks and the brakes are better then before the line blew up.

Thanks to everyone for the help. :cheers:

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Also, last night I was able to replace the brake line and bleed all of the brakes. No leaks and the brakes are better then before the line blew up.

Except there is a special procedure for bleeding the rear brakes when you have the height-sensing proportioning valve in the circuit, and if you don't have a factory service manual I'm sure you didn't follow this procedure. Which means if you lose the front brakes you may have NO brakes. Not good.

 

I transcribed the bleeding sequence from the FSM on here awhile back. I don't have it bookmarked but I hope you can find it with a search. If you're going to retain that "thing" in the back, you really have to be sure to bleed the system properly.

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Also, last night I was able to replace the brake line and bleed all of the brakes. No leaks and the brakes are better then before the line blew up.

Except there is a special procedure for bleeding the rear brakes when you have the height-sensing proportioning valve in the circuit, and if you don't have a factory service manual I'm sure you didn't follow this procedure. Which means if you lose the front brakes you may have NO brakes. Not good.

 

I transcribed the bleeding sequence from the FSM on here awhile back. I don't have it bookmarked but I hope you can find it with a search. If you're going to retain that "thing" in the back, you really have to be sure to bleed the system properly.

 

That "thing" (Load Sensing Proportioning Valve) is still used on Toyota and some Dodge pickups. But researching them, the design has been improved. My MJ valve is still functioning, and when I load the rear end down with the camper and a load of crap, the load sensing valve positively affects braking bias to the rear wheels, especially on a panic stop. I know Eagle sliced one and found the "safety" brake line orfice was faulty; maybe this was a defective valve. Eagle, ever find another one to slice? I will keep mine since it's still functioning, but am looking to replace it with the Toy version or install a Wilwood or other manually adjustable unit.

 

And X2 on the FSM brake bleed procedure. When I put rear disks on the back, I used the conventional bleed procedure because I was not aware of the FSM procedure. The brakes were definitely better than before, but after bleeding again following the FSM, the front/rear proportioning was much better.

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That "thing" (Load Sensing Proportioning Valve) is still used on Toyota and some Dodge pickups.

 

I don't think those are the only modern pickups to use something similar. I think most trucks do today.

 

I know Eagle sliced one and found the "safety" brake line orfice was faulty; maybe this was a defective valve. Eagle, ever find another one to slice?

 

I didn't slice the height-sensing valve. I sliced the front distribution block, and I found that the orifice that should feed the by-pass circuit was totally blocked (had not been drilled out). With this defect, the rear proportioning valve would have functioned normally under normal conditions, but it would also NOT be able to provide full-power (unproportioned) braking to the rear in the event of a front brake failure, due to there not being a viable by-pass circuit.

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I didn't slice the height-sensing valve. I sliced the front distribution block, and I found that the orifice that should feed the by-pass circuit was totally blocked (had not been drilled out). With this defect, the rear proportioning valve would have functioned normally under normal conditions, but it would also NOT be able to provide full-power (unproportioned) braking to the rear in the event of a front brake failure, due to there not being a viable by-pass circuit.

 

Sorry, my mistake, should have searched your previous posts. Since I've never had a front brake failure, and I still have the OEM MJ front dist block, I do not know if I have a design failure or not. And I did not research all, but I know the height sensing valve is still used in many modern trucks; I only stated the ones I was sure of. It's a good thing if it functions properly, and so far so good for me. Depends on what you use the truck for.

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It's a good thing if it functions properly, and so far so good for me. Depends on what you use the truck for.

It also depends on if it works. The one in my '88 worked ... until I had to make a panic stop. I guess I'm generally easy on brakes. The panic stop locked up all 4 wheels ... with 31x10.50 tires. It also exploded the height sensing valve, and they are no longer available from the factory.

 

I don't know about you, but for me exploding when I need the brakes most does not fall within the definition of "functions properly." If I can't trust it to work when I need it, I'd rather not have it there. My other MJs include an '86, an '87, another '88 and an '89. The one that exploded was about two years ago, so the ones I have are ALL older than that one was when it failed. Assuming the failure was due to age (the truck only had a bit over 100,000 miles on it then), they all have to be suspect. Therefore, as I get them on the road they will all be divested of that part.

 

I'm a careful driver, but I can't control other people. I need to know that my brakes will work if I have to make a panic stop.

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It's a good thing if it functions properly, and so far so good for me. Depends on what you use the truck for.

It also depends on if it works. The one in my '88 worked ... until I had to make a panic stop. I guess I'm generally easy on brakes. The panic stop locked up all 4 wheels ... with 31x10.50 tires. It also exploded the height sensing valve, and they are no longer available from the factory.

 

I don't know about you, but for me exploding when I need the brakes most does not fall within the definition of "functions properly." If I can't trust it to work when I need it, I'd rather not have it there. My other MJs include an '86, an '87, another '88 and an '89. The one that exploded was about two years ago, so the ones I have are ALL older than that one was when it failed. Assuming the failure was due to age (the truck only had a bit over 100,000 miles on it then), they all have to be suspect. Therefore, as I get them on the road they will all be divested of that part.

 

I'm a careful driver, but I can't control other people. I need to know that my brakes will work if I have to make a panic stop.

 

I suspect the height sensing valve might have been rusted pretty bad causing failure, or did the operating arm seals blow out? Agree, I want to depend on the various systems function as they should in all conditions and situations, expecially brakes. I'm looking at the toyota Tacoma valve, as detailed here: http://tinyurl.com/2y9a62

 

It's plumbed the same way as the MJ unit, looks much beefier, has it's own bleed screw, and uses basically a threaded operating rod for easy adjustment, i.e. no special tools like the MJ requires. I know my MJ valve won't last forever........

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