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rear brake proportioning valve


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so i blew a rusty old brake like out (and have an exhaust leak below the manifold on the 86. - 4 cyl) i figure i will wheel in to my mechanic and get it changed, -not so simple it looks like.

all the lines and bleeders are very , very rusty and he figures all will break including the proportioning valve. ---- nice to replace it but napa has never been able to get one.

I'm just wondering if anyone has run across this kinda prob before and any advice is , as usual, greatly appreciated.

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A quick search on this site will turn up innumerable discussions of this problem. I think it's safe to say that most of us recommend scrapping the rear height sensing valve entirely.

thank you eagle. ive never had much luck with the search feature and my mechanic went to check with his boss while i was there and was told that it was a necessary item and doom and gloom would happen (to me) if he wasnt able to save it.

could u give me a breif over-veiw of how to by-pass it ? --do u mean just have the lines go directly to the wheel cylinders ?

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:eek: FWIW... I've been thru this before , it needs to be said losing the valve isn't the end of the world but it can make braking a little skittery ( with no load in the bed ) til you get used to it . The usual way to by- pass is to install a T-fitting on the 2 lines coming from the master to the ride hieght sensing valve . ( this means you will have full pressure at all times / loads ) I guess a question might be did you have any problems with your "Pioneer " after the lift , because that mod would have negated the function of the perportioning valve unless you lengthened the( diff to valve ) rod ? .....Other stuff .... If you PB the hell outa the vavle conections , you shouldn't have a problem removing the fittings . Theres always a But you need to know the brake lines are a combination of metric and sae fittings , so you'll be buying a bunch of adaptors , AND if the axel lines are rusted out you can bet the lines along the frame to the master are toast as well .( good news is , it's easier to replace those with the gas tank out and you said before it needed to be replaced any way ) image_209027.gif

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A simple search with "rear proportioning valve" netted this 24 page result... Our search can be inaccurate, but it does work.

 

search.php?st=0&sk=t&sd=d&keywords=rear+proportioning+valve&start=0

 

Here is a couple from the first page:

viewtopic.php?f=2&t=23401&p=241535&hilit=rear+proportioning+valve#p241535

 

viewtopic.php?f=2&t=23265&hilit=rear+proportioning+valve

 

viewtopic.php?f=2&t=23189&hilit=rear+proportioning+valve

 

viewtopic.php?f=9&t=17968&hilit=rear+proportioning+valve

 

viewtopic.php?f=2&t=22773&hilit=rear+proportioning+valve

 

I too have been there and done that. I ran mine with out the valve and blocking the return line for a long time. no issues at all! Every single PU I have had locked up the rear brakes first, its never been an issue. Eliminating the rear prop valve and swapping in a XJ prop valve with the MJ's junction block under the master cylinder IMHO is our best choice.

 

CW

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The usual way to by- pass is to install a T-fitting on the 2 lines coming from the master to the ride hieght sensing valve .

This is incorrect. The usual way to eliminate the rear height sensing valve is the remove both the valve and both hard lines from the front distribution block to the rear. Plug the outlet on the bottom of the front distribution block, then run a new hard line from the "nose" outlet directly to the upper end of the rear flex hose.

 

I disagree with my colleague CWLONGSHOT about using an XJ front proportioning valve, though. I've been driving XJs for 22+ years and I was very active in NAXJA for many years before focusing more on MJs. I still have multiple XJs. One of the primary complaints about XJs is that the rear brakes suck. The reason they suck is that front proportioning valve.

 

What most XJ owners who address that problem do is open up the valve, remove the spring and O-ring from inside, and set the plunger to the full-forward position. What this does is disable the proportioning feature. That makes it the functional equivalent of the MJ system ... so why bother?

 

The XJ proportioning valve is a stupid device anyway. When it works (until it gets gummed up inside and sticks), what it does is prevent the rear brakes from doing anything unless you apply extreme pressure -- like in a panic stop. But premature rear lockup isn't a problem in normal driving, it's a problem in ... a panic stop. So what's the use of a device that eliminates the rear brakes most of the time yet provides no proportioning under the ONLY conditions when you need it?

 

Does not compute.

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so i blew a rusty old brake like out (and have an exhaust leak below the manifold on the 86. - 4 cyl) i figure i will wheel in to my mechanic and get it changed, -not so simple it looks like.

all the lines and bleeders are very , very rusty and he figures all will break including the proportioning valve. ---- nice to replace it but napa has never been able to get one.

I'm just wondering if anyone has run across this kinda prob before and any advice is , as usual, greatly appreciated.

Been through this... recently in fact.

 

Lines - replace em, no big deal except where they go behind the gas tank. I'm a perfectionist, so I tried to follow the routing on the body as much as I could. Took me about seven hours all told, and that was including 4-5 hours with no jackstands - just lying flat as I could under the vehicle. If I did it again it would likely take about 3 hours.

 

Rear height sensing valve - I'm one of those guys who actually likes to keep it. In fact, it saved my butt this morning... came over the brow of a hill and traffic was stopped right in front of me. No load in the bed. All four wheels locked up right before I came to a stop, I would have gone sideways into a parked car if I hadn't had either the rear height sensing valve or a manually adjusted one to replace it. Mine is entirely functional, even on my horribly rusted beater of an MJ. Guess I got lucky.

 

Bleeders - no big deal, pull and replace the rear brake cylinders. One of mine was so badly rusted it was somewhere between triangular and ovoid, the other had already been torqued off. Cylinders run about 10-15 bucks each, you will want two grade 8 1/4-20 bolts approx 1/2" long (I bought 1" ones and cut them down to length) to mount each one (since you mention rust your originals are probably pretty bad), so get four total.

 

You can do this entire replacement job in a single day if you are suitably motivated and have enough cold beer on hand.

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The usual way to by- pass is to install a T-fitting on the 2 lines coming from the master to the ride hieght sensing valve .

This is incorrect. The usual way to eliminate the rear height sensing valve is the remove both the valve and both hard lines from the front distribution block to the rear. Plug the outlet on the bottom of the front distribution block, then run a new hard line from the "nose" outlet directly to the upper end of the rear flex hose.

 

I disagree with my colleague CWLONGSHOT about using an XJ front proportioning valve, though. I've been driving XJs for 22+ years and I was very active in NAXJA for many years before focusing more on MJs. I still have multiple XJs. One of the primary complaints about XJs is that the rear brakes suck. The reason they suck is that front proportioning valve.

 

What most XJ owners who address that problem do is open up the valve, remove the spring and O-ring from inside, and set the plunger to the full-forward position. What this does is disable the proportioning feature. That makes it the functional equivalent of the MJ system ... so why bother?

 

The XJ proportioning valve is a stupid device anyway. When it works (until it gets gummed up inside and sticks), what it does is prevent the rear brakes from doing anything unless you apply extreme pressure -- like in a panic stop. But premature rear lockup isn't a problem in normal driving, it's a problem in ... a panic stop. So what's the use of a device that eliminates the rear brakes most of the time yet provides no proportioning under the ONLY conditions when you need it?

 

Does not compute.

 

I am with CW. I have the XJ prop valve and my braking is now great after eliminating the factory rear sensing prop valve and the MJ OEM distribution block.

 

Good luck. :cheers:

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:wall: :wall: i was looking for an icon of a guy pulling his hair out , but this will have to do.

 

if I'm understand this at all, - plug the line off the bottom front of the "distribution block" -(is this a "return line?), run the ONE line from the front of said block to a " T" at the rear, and run those lines to the wheel cylinders. thereby eliminating the rear proportioning valve.

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if I'm understand this at all, - plug the line off the bottom front of the "distribution block" -(is this a "return line?), run the ONE line from the front of said block to a " T" at the rear, and run those lines to the wheel cylinders. thereby eliminating the rear proportioning valve.

Not quite.

 

The "T" is already there in the back, on the axle. Run the new line from the front to the upper end of the rear flex hose, where it's clipped to the chassis.

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What I did with the Boss's truck is T the two lines together at the rear that would have went into the prop valve, take the third line off the T to the flex line to the axle..... same basic principle, one of the lines was emergency braking to the rear, the other was normal braking to the rear through the valve.... they both start out at the same point, I just ended them at the same point. Simple.

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What I did with the Boss's truck is T the two lines together at the rear that would have went into the prop valve, take the third line off the T to the flex line to the axle..... same basic principle, one of the lines was emergency braking to the rear, the other was normal braking to the rear through the valve.... they both start out at the same point, I just ended them at the same point. Simple.

 

:huh???:

 

Something doesn't sound right.

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"I too have been there and done that. I ran mine with out the valve and blocking the return line for a long time. no issues at all! Every single PU I have had locked up the rear brakes first, its never been an issue. Eliminating the rear prop valve and swapping in a XJ prop valve with the MJ's junction block under the master cylinder IMHO is our best choice." - CW

 

this makes sense to me so this is what ive done - except used the mj distribution block as it was laid out in the pics. ------ will road test tomorrow and if its good -then great, -if not - good, will put in the xj distribution block. ----

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What I did with the Boss's truck is T the two lines together at the rear that would have went into the prop valve, take the third line off the T to the flex line to the axle..... same basic principle, one of the lines was emergency braking to the rear, the other was normal braking to the rear through the valve.... they both start out at the same point, I just ended them at the same point. Simple.

 

:huh???:

 

Something doesn't sound right.

 

The main brake line to the rear goes into the valve section of the rear valve, the second line going to the rear is for emergency stops, and will send braking to the rear bypassing the valve... both should have full fluid in them, and t-ing them together just makes you have two fluid lines going to the rear, one that will provide pressure when the pedal is used normally, and one that provides presssure in a panic stop, T-ing them together and then going out to the axle just eliminates the rear valve, and full pressure goes to the rear brakes all the time instead of just in a panic, and will be doubly used if there is a panic. Does that make sense? Maybe I can draw it up in paint if I need to. This way you don't have to mess with the front valve or eliminate any lines, or block anything. Works fine too!

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The main brake line to the rear goes into the valve section of the rear valve, the second line going to the rear is for emergency stops, and will send braking to the rear bypassing the valve... both should have full fluid in them, and t-ing them together just makes you have two fluid lines going to the rear, one that will provide pressure when the pedal is used normally, and one that provides presssure in a panic stop, T-ing them together and then going out to the axle just eliminates the rear valve, and full pressure goes to the rear brakes all the time instead of just in a panic, and will be doubly used if there is a panic. Does that make sense? Maybe I can draw it up in paint if I need to. This way you don't have to mess with the front valve or eliminate any lines, or block anything. Works fine too!

But ... you have two, parallel lines doing the job of one. That may be okay of you live in Arizona, but in my neck of the woods if I can have ten feet of brake line subject to rust rather than twenty feet, I'm all over it.

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