inside of the MJ combo brake valve

Eagle was kind enough to cut one open to help answer the riddle of what on earth it does. Unfortunately, his unit has cropped up a couple questions that are more troubling. I'll let him tell you the story when he gets back.

Jeep on!

--Pete

another reason that I'm gonna put in an XJ propo valve when I redo my brakes.

Interesting. I was just looking in the Jegs catalog, At buying some kind of adjustable one, to improve my braking perfromance. It seems that when I use my brakes on the trail they seem to fadereally bad, especially when I need them to work.

Patrick

I like how they seemd to have f'd up boring that thing.

Makes me feel REAAAAAAAAAAAAAAAL safe.

Interesting. I was just looking in the Jegs catalog, At buying some kind of adjustable one, to improve my braking perfromance. It seems that when I use my brakes on the trail they seem to fadereally bad, especially when I need them to work.

 

 

 

 

Patrick

well, my brake lines are pretty much shot. I was going to do a complete system retrofit on the MJ in the spring. 96+ Booster, XJ propo valve (the rear bed one is already out of the loop) and an adjustable propo valve for the back if the rear drums are too touchy, it's likely they will be, especially with the new dual diapraghm booster. That way I only have to run one brake line to the back, and have one less failure point.

I like how they seemd to have f'd up boring that thing.

That's the disturbing finding.

The circuit that has the outlet at the "nose" of the junction block is always active, and that's the line that feeds through the rear load-sensing proportioning valve. My concern was that when in override/bypass mode this circuit might be blocked. That is not the case, it will send pressure to the rear even when the front brakes fail. So from that perspective, my dissection was valuable.

But notice the vertical bore from the rear circuit on the bottom of the junction block. This is supposed to be the bypass circuit that sends full pressure to the rear if the front brakes fail, rather than proportioned pressure. When the front brakes fail, that sliding piston on the top moves to the right, opening up the bypass circuit. Except ... the hole isn't bored all the way through to the piston bore. That's not an optical illusion ... the hole stops about 3/32" short of making a connection. If the front brakes fail, this valve would always allow proportioned ear brakes, but would never have gone into bypass mode.

Now I want to get a few more of these blocks, to see if this one was the odd man out or if they were all defective.

Pete -- thanks for putting the picture up.

Yeah, I was wondering if that was a manufacturering flaw, or if it was made like that.

patrick

I think the one in my '87 is like that too. I have just the front and rear bottom lines hooked right now. (my hoses are fairly recent and I have '87 YJ hoses for extra length)

I definitely do not claim to be an engineer, but if air was to get in the line that is supposed to work if the front brakes fail, how do you get fluid through to push out the air. And if all are made like the picture it would not be possible at all. Just my two cents worth.[/i]

I have one off a 98 xj that you can have if you want to cut it open. I'd actually be interested in seeing just how much different it is, as the one from my xj, versus the one I reused in the MJ have different fittings coming in from the Master Cyl.

I'll also see if I still have my 93 one around, just for schitts-n-giggles....

Be interesting to see just how wide-spread this issue may be.

Digging up bones..... was wondering if Eagle had anymore prop valves cut apart..??

I was hoping the one that was pictured was just a freak...

I pasted the pic to paint and drew what I understand is the right lines, please if its wrong let me know and I'll fix it.....

Image Not Found

Are the front brake lines right on the drawing??

Another thing I was pondering... if i shortened the height rod wouldn't it increase braking force at the rear?

I am not sure how the auctual valve works in the rear. I wondered about it.. then I decided it was just easier to do away with it. and only have one brake line to worry about rotting :brows:

I am not sure how the auctual valve works in the rear. I wondered about it.. then I decided it was just easier to do away with it. and only have one brake line to worry about rotting :brows:

I have a 96 xj booster I'm going to swap in. I'm just going to have to wait before I can get the right tools and parts together before it happens. I have replaced a few small lines on that valve anyways so I'm going to run it until everything is together to swap the xj's booster in. I live in NC so rust isn't on the top ten here.

I just want to make sure I have everything right so I only have to do it once when it happens..

+ I'm hoping Eagle will see this and post up more pics of cut away valves

So what is this common failure/problem with MJ brake systems??? I've experienced some funk with everything since I removed the right front calliper... do our brakes like to act up?

So what is this common failure/problem with MJ brake systems??? I've experienced some funk with everything since I removed the right front calliper... do our brakes like to act up?

From what I gather bleeding them, the rear height adjusting valve is a POS, rusting lines, crappy booster, and parking brake cables breaking...

The biggest problem would be that height adjusting valve in the rear...

But hey at least their not ABS,,,,,, gezzz I hate to see what that would be like........

Another thing I was pondering... if i shortened the height rod wouldn't it increase braking force at the rear?

Nope.

When you add load to the bed, the axle stays where it is and the truck bed drops. The link rod pushes the arm on the proprtioning valve UP.

If you shorten the rod, you pull the arm on the valve DOWN, which reduces your braking. If you want more brakes, make the rod longer.

Another thing I was pondering... if i shortened the height rod wouldn't it increase braking force at the rear?

Nope.

 

When you add load to the bed, the axle stays where it is and the truck bed drops. The link rod pushes the arm on the proprtioning valve UP.

 

If you shorten the rod, you pull the arm on the valve DOWN, which reduces your braking. If you want more brakes, make the rod longer.

Hey Eagle you have anymore valves cut open??

Did I get the line labels right?

You got the labels right. I don't have any other MJ junction blocks to cut open, but I did slice an XJ combo valve. I wanted to see how it works. Even after cutting it open and studying it, I still don't understand how it provides any proportioning. I *WAS* going to replace the MJ junction block with an XJ combo valve. Now that I have looked inside the XJ unit, I have given up that notion. I'll stick with the MJ junction block and bypass the rear proportioning valve. I can deal with excess brakes in the rear much better thah I can deal with no brakes in the rear.

I cut my prop valve in half(mines an '89) and it looks exactly like all the pics so far. Completely useless as a failsafe. Maybe jeep will issue a recall ;)

how did you guys cut it in half? A mill or bandsaw? Was thinking if you milled it down a few .001's at a time it would show if theres any bore going all the way up. and if I removed the bypass line and capped it, then open the front bleeder it should shoot fluid out of that spot on the prop valve.. If not I think it would be safe to say that whole line is unneeded.... and then I could just cap it off at the tee in the rear and not have to f%