MJ Rear Porp Valve...

I have that rear porp valve for my rear brakes on my MJ... I have been doing some heavy mods on my MJ and now this thing just gets in the way. I know there is a write up somwhere on how to replace it but I have yet to find one. I am looking for somthing simple... I dont ever haul hardly anything in this truck either, its kinda an all street MJ...

I don't think it was exactly a write-up, but CheapXJ mentioned it in a post recently.

Rip it out. Then go to the front combination valve, located just under/next to the master cylinder. Trace the steel line that comes out of the NOSE of the combination valve in the front all the way to the rear. Remove that line -- you won't need it. Reroute the remaining rear brake line to connect directly to the hose from the axle.

Replace the MJ front combination valve with an XJ combination valve.

Bleed the brakes.

Done.

Actually, first off I should ask is this a good idea if I am going to rear disc's??? Also I know how to tell on the newer models by the comp on the fender, but how do I know if I have ABS on my 89??

If you remove it, you'll need to plug the rear return port on the combo valve, or swap it out for an XJ valve, like Eagle said. But, the return line is on the bottom of the valve (the front one), not the one coming out of the front of the valve. That's the rear output line...cap that one off and you'll have 0 brakes in the rear. Found that one out by trial and error
HTH,
Jeff

Jeff in VA said:

If you remove it, you'll need to plug the rear return port on the combo valve, or swap it out for an XJ valve, like Eagle said. But, the return line is on the bottom of the valve (the front one), not the one coming out of the front of the valve. That's the rear output line...cap that one off and you'll have 0 brakes in the rear. Found that one out by trial and error
HTH,
Jeff

Click to expand...

Jeff, I disagree. There is NO line coming out of the "nose" of the XJ combo valve. How could the "nose" line be the primary output on the MJ yet not exist on the XJ?

I don't specifically know the inner circuitry of the two different valves, but the Xj one has that nut on the front that you unscrew to access the spring inside the valve body, whereas the MJ one has the hard line. If you compare them side by side, the entire valve body is slightly different, and the MJ valve has only one line to the rears, which passes thru the rear prop valve, and then splits off to each rear wheel cylinder. I've got a few spare XJ valves that I was going to use in place of my original, but so far the plug fitting in the bottom of the valve is working quite well. IIRC it's a 3/16"-ish sized plug.
Jeff

Hmmm ...

It appears that my understanding of how the front valve in the MJ works was incomplete ... and still is.

In the XJ, it is a "combination" valve, serving as both the brake system warning sensor and as the proportioning valve. The "nose" has a removable plug, behind which is a spring-loaded plunger with an O-ring. It's clear that there is some functionality going on inside there.

On the MJ valve, there is no removable plug in the front. I believed that the rear circuit (the entire valve, in fact) acted simply as a junction block, serving as the brake system warning device but not providing any proportioning function, since on the MJ that's done at the rear by the load-sensing valve.

So far, so good. But I just read the MJ FSM and it appears the front valve does have an additional function -- and I can't figure out how it works because there are no diagrams. The thing isn't supposed to be serviced, only replaced. Note that in the XJ FSM this device is referred to as a "combination" valve, whereas in the MJ FSM it is referred to as a "by-pass differential" valve.

Here's what it says:

MJ FSM said:

VALVE OPERATION
The by-pass pressure differential valve activates the brake warning lamp in the instrument panel if a pressure loss in the front or rear hydraulic system should occur. This feature is provided as a means to alert the driver of a system malfunction.

The by-pass pressure differential valve contains a piston and a plunger-type switch. If a pressure loss occurs in either the front or rear hydraulic brake system, a system differential of 483 to 2068 kPa (70 to 300 psi) will cause the piston to move toward the low pressure side of the valve.

Also, if the valve senses a loss in pressure in the front system, it will direct fluid pressure to the rear wheel cylinders, by-passing the height-sensing proportioning valve.

Click to expand...

The last sentence was put in bold by me, for emphasis, because this is new information for me. I have no idea how the valve does this, but it suggests that the second line that I (and other folks) have always thought of as a "return" line is NOT a return line, but a by-pass line that overrides the normal proportioning function.

Since I don't know how to determine how this valve works internally, the best advice I can offer for an MJ with rear drums would be to take it out and use an XJ combo valve. In the past I believed you could just plug the front (nose) outlet and that the MJ valve was open internally, just acting as a distribution block. Now I don't know.

For an MJ with rear discs, it may make sense to use an XJ combo valve and modify it the same as would be done on an XJ with rear discs, or to use a valve from a ZJ that was equipped with rear discs from the factory.

I think I need to get a couple of MJ valves from the u-pull yard, and cut one open to see what makes it tick.

I have moved that load proportioning valve to a frame rail and am going to use a control cable to operqte it so I cna use it to dial in a little more or less rear brakes, like on long decents would be nice to have more rear less front

I deleted the rear valve completely and plugged the front return port. Seems to work just fine with my 190k mile old booster, 78 Ford axles and 33" tires.



(notice that this was my second try at it )
Jeep on!
--Pete

ComancheClub.com

Jeff in VA said:

I don't specifically know the inner circuitry of the two different valves, but the Xj one has that nut on the front that you unscrew to access the spring inside the valve body, whereas the MJ one has the hard line. If you compare them side by side, the entire valve body is slightly different, and the MJ valve has only one line to the rears, which passes thru the rear prop valve, and then splits off to each rear wheel cylinder. I've got a few spare XJ valves that I was going to use in place of my original, but so far the plug fitting in the bottom of the valve is working quite well. IIRC it's a 3/16"-ish sized plug.
Jeff

Click to expand...

Looks like you and PeteM did the same thing. But ... the outlet that both of you plugged is where the rear brake outlet is on the XJ combo valve. And you wrote that "... the MJ valve has only one line to the rears, which passes thru the rear prop valve, and then splits off to each rear wheel cylinder." That's not the way mine are set up. There are two lines from the front "pressure differential" valve to the rear proportioning valve. One comes off the "nose" of the front valve (that's the one you retained), and one comes out the bottom ( that's the one you and pete plugged, but which IS the rear brake outlet on the XJs.

I don't suppose one of you lads would be interested in opening a front bleeder screw to simulate a front brake failure, and then slamming on the brakes (in the driveway, at low speed) to see if you still have rear brakes? I don't have a clue how the front valve accomplishes the by-pass function described in the FSM, but if it has that function built into it, it seems to me that if you lose your front brakes the front valve is going to try to "by-pass" the circuit you're using, and send the fluid through the circuit you blocked off. Which would leave you with no brakes. I think we can agree that's probably not "a good thing."

This makes me think all the more that CheapXJ did the right thing by just swapping in an XJ combo valve.

Eagle said:

"... the MJ valve has only one line to the rears, which passes thru the rear prop valve, and then splits off to each rear wheel cylinder."

Click to expand...

Sorry, I didn't type that too well. There are 2 lines to the rear prop valve, but only one of them pushes fluid when the pedal is depressed (the front one out of the nose). The other, which we collectively have been calling the "return" line, comes off the bottom front and runs to the valve as well. When I bypassed the rear valve on the first attempt, like Pete I capped the front line off, and got nothing out of that lower line. So I reattached the front one and plumbled it to the distribution block @ the rear axle and capped the lower port, and viola....had rear brakes.

Eagle said:

I don't suppose one of you lads would be interested in opening a front bleeder screw to simulate a front brake failure, and then slamming on the brakes (in the driveway, at low speed) to see if you still have rear brakes? I don't have a clue how the front valve accomplishes the by-pass function described in the FSM, but if it has that function built into it, it seems to me that if you lose your front brakes the front valve is going to try to "by-pass" the circuit you're using, and send the fluid through the circuit you blocked off. Which would leave you with no brakes. I think we can agree that's probably not "a good thing."

Click to expand...

I'm inclined to think the same way.....I may just have to install one of those XJ valves I have just to be on the safe side. I do know that the MJ prop valve isloates a bad rear circuit, because I blew the rear line above the axle a long time ago and it drained the rear resivoir on the MC, but the front maintained the proper level and I had marginal braking at best LOL.

This has proven to be a most informative thread, Eagle. Thanks for your analysis
Jeff

Jeff in VA said:

This has proven to be a most informative thread, Eagle. Thanks for your analysis
Jeff

Click to expand...

Second that. I too have been thinkin' about tossing mine.

Okay now i am in the same boat with my MJ. My rear return line (bottom on the front distribution block/proportion vavle) was rusted and split wide open and I didn't even notice braking difference. Although, that is how I purchased the truck anyway! So now what I plan to do is bypass all that crap in the rear and just get a standard 't' fitting to go on the rear axle and connect it to both rear cylinders and then the output line on the nose of the front distribution block. Does that sound right?

Oh yeah; 1987 MJ, 4x2, totally stock.

Thanks.

Burley

Burley said:

Okay now i am in the same boat with my MJ. My rear return line (bottom on the front distribution block/proportion vavle) was rusted and split wide open and I didn't even notice braking difference. Although, that is how I purchased the truck anyway! So now what I plan to do is bypass all that crap in the rear and just get a standard 't' fitting to go on the rear axle and connect it to both rear cylinders and then the output line on the nose of the front distribution block. Does that sound right?

Click to expand...

Based on Jeef's and Pete's experience, that will work.

But it doesn't resolve the issue I am worried about: The FSM says that if you lose the front brakes, the combo valve (differential valve) will bypass the load sensing proportioning valve and allow full brake pressure to the rear wheels. The only way it can do that is by using that other line -- the one you're talking about plugging, and which Jeff and Pete did plug.

My concern is that if you lose the front brakes, the mod you are considering would leave you with NO brakes. I would plumb it the way you're talking about, but use an XJ combo valve in place of the MJ differential valve.

Most informative indeed.

It comes to mind that I've never heard of a 'return line' when referring to brakes, where did that term come from in this instance? I seem to believe the theory that it is in fact a bypass line to allow rear brakes should the front brakes fail. This being the case, you would have no brakes should the front brakes fail.

Bounty Hunter said:

Most informative indeed.

It comes to mind that I've never heard of a 'return line' when referring to brakes, where did that term come from in this instance? I seem to believe the theory that it is in fact a bypass line to allow rear brakes should the front brakes fail. This being the case, you would have no brakes should the front brakes fail.

Click to expand...

That would be correct. I believe. Eagle is conducting an experiment that SHOULD put light on the whole thing.

Stay tuned.

The way the valve is set up is that the extra line is a "sensor". If you don't have front brake pressure, the plunger inside slides to the lower pressure side, and bypasses the load valve.
Both lines (when operating properly) have pressure at the back when you step on the brake. If you are trying to bleed the rear brakes, and can't get any pressure, you need to loosen the line in front (marked rear output in the photo above) as any air will keep the rear brakes from being applied.
It actually releases the pressure going to the front brakes and makes the valve move over so the level regulator in the back does it's job.
Frankly, it's a POS system and you should get an adjustable valve from Summit (I think it's Sum 035 but can't quite recall) so that all 4 discs can be front/rear bias adjusted and you can avoid rear skid on wet pavement.

Well
I have removed the rear valve on both of my MJ's
all I did was run both lines into the existing "t" valve and that was it
brakes have allways worked fine.
So I know that you dont need to cap off or change anything up front

and the braking of the trucks is just as good as before

And I would not have thought it would be a return line?
I thought it was a back up "safty" line if the rear valve quit working?

Comanchedude said:

Well
I have removed the rear valve on both of my MJ's
all I did was run both lines into the existing "t" valve and that was it
brakes have allways worked fine.
So I know that you dont need to cap off or change anything up front

and the braking of the trucks is just as good as before

And I would not have thought it would be a return line?
I thought it was a back up "safty" line if the rear valve quit working?

Click to expand...

This is what I'm considering doing to my '88, I just pulled the rear axle in favor of a SOA and will soon be looking at pulling that rear brake thingamajigger above the axle.

Would you be able to post a pic of how you ran both lines from the front into this existing T connector?

I've also got a '93 XJ proportioning valve here if it'll be the best route to go, but I like the idea of the rear brakes still working should the front lose pressure.