'89 MJ brake question.

I'm doing lines on my MJ. I have a bugger of a question. I have 2 lines that run back. One goes to a combination valve to split to each wheel, and the other line goes to a bracket with what looks like a combination valve, then its connected to another bracket, with a horizontal rod that connects to the back plate on my differential. What the hell is it, what does it do and do I need it? Ill throw up some pictures as soon as my digital camera is finished recharging. I spotted another MJ at the A&P down the road so I looked under it and lo and behold, it had the same thing. spliced into the brake lines. WTF is it? Ive got the D35 by the way.

MoparRobbie -

There was alot of coverage on this subject some time ago, and here is the post about the rear porportioning valve -

http://comancheclub.com/forums/viewtopi ... ning+valve

I redid my lines with only one line from the front, to the back and used the "T" from the rear to tie the lines togetter in the front, and just cut off the rotted out rear valve, also the parts to rebuild it don't exist anymore.

Only problem I have is, with no load in the back, and on a panic stop, my front brakes lock up.

Also, read up on what Eagle had wrote up, very good explation on this subject. :popcorn:

There's what's left of mine :D

Jeff

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ok, so can someone toss up a quick instruction on how to eliminate it from my braking system. I need to do the entire set of lines on it, so what all do I need to do? Just block it off at the master cylinder and split the lines on the axle like they are already?

Ok, I have a halfgood idea of what I need to do. Take the T split where the main line for the brakes splits off into the proportioning valve and run it from the master. tie it into the main line for the brakes and keep the combination vavle at the back that splits the lines to each wheel. Ok. If there's a problem with that, let me know pls.

If you're redoing lines, I would get a prop valve from an XJ to replace the MJ combo valve. Then simply route a single line from the XJ output to the rear axle. You know, like normal vehicles have.

Find yourself an XJ proportioning valve and run a single line to the rear. Simple and clean.

Haha beat ya to it. :thumbsup:

If you're redoing lines, I would get a prop valve from an XJ to replace the MJ combo valve. Then simply route a single line from the XJ output to the rear axle. You know, like normal vehicles have.

Pete, for shame. Even you should know that these things aren't normal vehicles, decent folk don't drive these things! :P

Ill track down an XJ prop valve. does it fit in the same spot as the MJ combo valve? If I can't track one down by wednesday, I'm doing a straight line from the front with the MJ combo and a bypass for the rear prop valve. its gotta be done.

If you have to use the MJ one, this is a pic of mine:

The lower port is plugged.

Pete, I just yanked the combo valve out, lo and behold the @#$%er had the line welded solid. the rear prop valve isn't even working! guess one of the PO's had a problem with it and removed it from the eqn. so I've got no problems at all now! thanks everyone

If I can't track one down by wednesday, I'm doing a straight line from the front with the MJ combo and a bypass for the rear prop valve. its gotta be done.

The XJ proportioning valve will directly replace the MJ valve. If you decide to go with the MJ valve, be VERY careful, and refer back to my old post with the photo of the dissected MJ valve.

On the XJ, the "nose" of the proportioning valve is a rubber cover over the stem of the proportioning plunger, and the line to the rear brakes come out the front bottom outlet of the valve body.

On the MJ, the line out the nose of the valve body (actually, in an MJ it's just a junction block, it doesn't function as a proportioning valve) is the line to the rear proportioning valve, and the line out the forward bottom outlet (corresponding to the XJ rear line) is the bypass line that gets actuated only when you lose pressure to the front brakes.

If you retain the rear line out of the nose of the MJ combo valve, you're okay. Block off the outlet at the front bottom of the unit. However, because the XJ valve feeds the rear brakes from the outlet on the underside of the valve body, it's a natural tendency to think that's the outlet to use with an MJ combo valve.

Bad guess. The outlet on the bottom is the safety override, and it allows fluid passage only if you lose the front brakes. Worse, if you look at the photo of the valve I cut open, you'll see that it wasn't manugactured correctly and would NEVER allow fluid to flow to the rears. I suspect the one in my '88 has the same defect, because since the rear proportioning valve blew out and I capped that line I have had NO rear brakes. (Of course, I haven't lost the fronts since then ... thank God ... so there really shouldn't be any fluid flowing in that line.)

If you keep the MJ combo valve, connect your rear brakes to the outlet in the NOSE of the valve body.

Here endeth the daily lesson.

http://comancheclub.com/forums/viewtopi ... portioning

Only by a minute, Pete, only by a minute :cheers:

I'm interested in doing this but what if i took the easy way out and put this in the rear

http://www.4x4wire.com/jeep/projects/co ... /valve.jpg

Ok, I have a question. What about leaving everything alone (going with the "if it ain't broke don't fix it" theory) and swapping out axles to full width and rear disc's? The rear proportioning valve was designed for drum brakes right? I know some of you guys on here have switched to rear disc, so what did you do? Just leave it alone or take it out? I'll also be running a Ford 1 ton master cylinder, at least that's what I was told to run. Just let me know what you guys think. Thanks, Shawn. 8)