Mj Brake Proportioning Valve

[jordan]

Ive got an 88 Comanche,4.0,8ft bed,4wd with 56k on it.no rust and in very nice condition.I know there has been discussion on bypassing the proportioning height/weight valve at the rear of the truck and some discussion on the distribution block on the front.Approaches vary however and I'm hoping someone has a step by step procedure to deal with this?.The stock suspension is in place so theres no lift issues with the rear valve.

 

I have no problem eliminating the rear valve but if its still good and just the front valve needs to be cleaned/recentered/or even replaced.I would just as soon keep it stock.

 

If I bypass the rear valve,I would just block the bottom forward fitting on the front block and not source a YJ block.Doesnt seem necassary.Both of these parts are out of the truck currently so some direction would be much appreciated......Thanks ,Jordan

[Eagle]

There is no front "proportioning valve" in the MJ. That distribution block beneath the master cylinder also acts as the proportioning valve in the XJ and ZJ, but not in the MJ. The XJ units have the "nose" occupied by a plug, behind which there's a heavy spring and a slider with an O-ring on it. None of that is present in the MJ. In the MJ, that unit is a distribution block, and also provides for a bypass to send full braking pressure to the rear wheels (by-passing the height sensing valve) in the event of a failure of the front brake circuit.

 

The simple way to eliminate the rear height sensing valve is to disconnect the rear axle flex hose from it, run a new hard line from the "nose" of the front distribution block directly to the rear flex hose, and to plug the forward/lower output on the distribution block. That's it -- done.

 

The cautionary note here is that the bypass will then result in the rear wheels always getting full braking pressure. To me, that's a good thing. However, you must always be aware that when the bed is unloaded, there will be a tendency for the rear wheels to lock up prematurely. If you're lot looking out for it, this will result in a spin-out. It's not an issue for me because I learned to drive before cars (and light trucks) had proportioning valves, so I'm accustomed to modulating braking with my right foot. For people who don't know anything other than proportioned braking and ABS, it takes some getting used to ... and some drivers can never adapt. For those cases, I recommend installing a Wilwood adjustable proportioning valve in the rear circuit.

 

BTW -- I do NOT recommend using an XJ or ZJ proportioning valve in an MJ. First, they tend to get gummed up, leaving you with NO rear brakes. Second, the way they operate is IMHO exactly the opposite of what they should do. What they do is delay the flow of brake fluid to the rear wjeels until the pressure is fairly high. This means that most of your daily driving will use only the front brakes. (This explains why the rear brakes on my wife's 2000 XJ still had over 60% of the lining left when I opened them up at 100,000 miles.) The rear wheels don't see any braking until you really stomp on the peddle -- which is a panic stop, which throws most of the weight onto the front wheels, which is exactly when you DON'T want to suddenly be dumping pressure to the rear wheels. Even with no proportioning valve, I've never spun out my MJ. I DID spin out the 2000 XJ -- on a highway at 65 MPH when some idiot came up an entrance ramp and cut me off. It was a miracle the XJ only spun out and didn't flip over.

 

By contrast, my '88 XJ hasn't had any rear brakes for two or three years. One of these days I need to disassemble the proportioning valve and clean it out.

[Tactical Bacon]

If you pull the height based brake proportioning valve off your MJ, let me know. I'll buy it and the rod. Always good to have a spare.

[cruiser54]

Just removed the load sensing valve and plumbed things as Eagle suggested.

 

Brakes have NEVER been better.

 

I ordered an adjustable proportioning valve for the rear from Jeg's before I did this. 

 

Jury is still out on this, but I may just leave it as is and skip the valve.

 

FWIW, I do have the dual diaphragm booster and have had it for a while. . 

[Dillithium]

I feel like this post needs to be moved to the DIY section, this is the guide I followed and it helped me tremendously. I did it without the dual diaphragm booster and I found the braking to be good enough(with new pads and new shoes). That is, until I drove around with 700 lbs of cargo. That made my braking a much scarier experience, which is why I'll be swapping in my WJ master and booster soon.

 

This is gold for people with a broken prop valve.

[Incommando]

Dual diaphragm XJ booster and no rear valve equals amazing brakes! Just did it to mine.

[drase]

The stock suspension is in place so theres no lift issues with the rear valve.

 

 

 

 

 

 

I have a 3" suspension lift on mine and the extender from the the rear brake valve keeps popping off the rear diff. How would I fix this?

[Strokermjcomanche]

Make a 3 inch longer rod .

[1stDeuce]

Reviving a older thread that I find Interesting.  I was planning to do a XJ or TJ prop valve swap on my Comanche since the prop valve is unhooked and fairly crusty anyway...

 

Since this truck will be my wife's daily driver, I don't want to just sending full pressure to the rear, since she'll be driving it unloaded most ot the time. 

 

Eagle, you recommend against a prop valve, but your understanding of how a prop valve works is completely upside down...   Prop valves work off pressure, not flow.  It should allow full pressure UP TO a set point, at which it knees off at a percentage of the input pressure.  The idea is that under low to moderate braking, it's fine to use full  braking at the rear, but as you stop faster, weight transfer lightens the back of the vehicle, requiring less pressure to the rear to prevent lockup.   The prop valve is supposed to make sure the rear always locks AFTER the front.  (Fronts always get full MC pressure)  It's also important to note that drums take LESS pressure to lock than discs normally, so a prop valve from a drum brake vehile will be under-braking the rear on a disc vehicle.   Shorter wheelbase vehicles also tend to knee at a lower point, since they are more prone to weight transfer than longer wheelbases.

 

The Wilwood adjustable pressure regulator that has only one inlet and outlet is often referred to as a prop valve, but it is NOT.  It only functions to limit pressure to an adjustable set point, meaning it'll allow full pressure up to a certain point, then limit it to that pressure no matter how high input pressure goes.  Exactly like a pressure regulator used on an air comprssor output line.  That's great if all your stops are under the same condition, but not so good for loaded vs unloaded, or wet or slick roads vs. dry ones...  

 

SSBC and Jegs both offer adjustable proportioning/distribution valves that offer a true knee point, with continued rising pressure after the knee point.  This prop/distribution valve functions exactly like the factory prop valve, but offers an adjusable knee point.  This is a much better solution for a daily driver!!  Additionally, the distribution valve has the ability to throw the brake light if one circuit fails, and keeps the two circuits separate so failure of one does not mean failure of all. 

 

As to the lack of wear in the XJ rear brakes, it may be true that the XJ is under-utilizing the rear brakes, but it probably has a lot more to do with adjustment of the drum unit than pressure.  If the self-adjuster seizes, or one never uses the park brake to adjust it, then you can get to the point of needing more volume than the master can supply, and if that happens, your rear brakes become fairly ineffective.   Unless your wife was a real hard braker, I would expect the rear drums to last quite a bit longer than the front discs anyway... Disc brakes have much higher pad clamp force than drums for the same amount of stopping torque, and usually less pad area, so they wear out faster.     

 

I guess I should put a build thread on here... I've been doing it over at Expedition Portal, since that's where I was hanging out most of the time, and this truck is our weekend getaway ride!  :)

Chris

[Eagle]

Eagle, you recommend against a prop valve, but your understanding of how a prop valve works is completely upside down...   Prop valves work off pressure, not flow.  It should allow full pressure UP TO a set point, at which it knees off at a percentage of the input pressure.  The idea is that under low to moderate braking, it's fine to use full  braking at the rear, but as you stop faster, weight transfer lightens the back of the vehicle, requiring less pressure to the rear to prevent lockup.   The prop valve is supposed to make sure the rear always locks AFTER the front.  (Fronts always get full MC pressure)  It's also important to note that drums take LESS pressure to lock than discs normally, so a prop valve from a drum brake vehile will be under-braking the rear on a disc vehicle.   Shorter wheelbase vehicles also tend to knee at a lower point, since they are more prone to weight transfer than longer wheelbases.

I understand what a brake proportioning valve should do. And that's not how the XJ proportioning valve works. It works exactly the opposite of the way you say a proportioning valve should work. I agree that's how they should work -- but that's NOT how the XJ proportioning valve works. I've had them apart, and I cut one in half to verify how it works. It completely blocks flow (and pressure) to the rear brake circuit until the pressure in the system gets pretty high, at which point the plunger overcomes the resistance of that big spring in there and allows the rubber cup seal to move forward and open the inlet to the rear circuit. In normal, day-to-day driving with light to moderate brake pressures, the rear brakes on an XJ do not do anything.

 

With the XJ proportioning valve, once the plunger moves forward and the rear circuit opens, the pressure to the rear is the same as the pressure to the front. Always.

 

And that's why I don't like the idea of the XJ proportioning valve in the MJ. If the bed isn't loaded, the tail of an MJ is relatively lighter than that of an XJ. You drive it around empty, and you get used to the rear always staying where it should be -- following the front. Then one day you get into a panic stop situation, you stomp on the brakes hard, the rear circuit opens up, and the rear wheels lock up. Spin-out.

 

Yes, the same braking with NO proportioning valve would also lock up the rear brakes. But -- if you don't have any proportioning valve, you know how the brakes are going to react so you're not taken by surprise when all of a sudden you have rear brakes.

[1stDeuce]

Eagle, I'll have to take a look myself, but having the prop valve function like that would be completely negligent, and XJ's would be spinning out of control in panic stops everywhere.  Either your inspection was in error, or someone really messed up that prop valve.  As you say, a valve that functioned like that would kill people, and to the best of my knowledge, XJ's don't have issues locking the rear in a panic stop, which would result in a spin.  Quite the opposite, actually.   (Not enough rear brake in panic stops.)  ?? 

 

Well, I'll take a look at the one I get before I put it in I guess...  If it's that messed up, I certainly don't want it in my truck!!  Any idea what year XJ yours was?  

[outlaw star]

Eagle, will this help you better explain it with?

[Eagle]

Well, I'll take a look at the one I get before I put it in I guess...  If it's that messed up, I certainly don't want it in my truck!!  Any idea what year XJ yours was?

They're all the same.

 

Just unscrew the bolt-like thing in the "nose." Be careful -- there's a strong spring inside. Once it's open, you'll see the plunger and the cup washer.

[Eagle]

Eagle, will this help you better explain it with?

Nope -- that's the MJ distribution block. It has no proportioning function. I did the same surgery on an XJ proportioning valve, then I misplaced it before I could photograph it. I guess I'll just have to do it again, because the question keeps coming up.

[Eagle]

Update: I found the dissected XJ proportioning valve. And, I'm ashamed to admit, it does NOT appear to work the way I thought it did. In fact, after studying it more closely, I find it's a bit more sophisticated than I thought, and ... I have no clue how it works at all under normal braking. (Which may be why the hot tip in NAXJA was to remove the cup washer and disable the dang thing).

 

I'll post photos in a day or two.

[phenryiv1]

 

SSBC and Jegs both offer adjustable proportioning/distribution valves that offer a true knee point, with continued rising pressure after the knee point. 

 

Which valves on the Jegs site are correct for the MJ?  In particular in conjunction with a WJ booster/MC swap, if that matters.

[Crash]

If they are adjustable, than in theory they will work.. 

[1stDeuce]

Which valves on the Jegs site are correct for the MJ?  In particular in conjunction with a WJ booster/MC swap, if that matters.

 

This one would be my first choice if I was spending someone else's money:  P/N 63025

There's also a SSBC version that's P/N 884-A0730.  

Both willallow you to adjust the "knee" point at which it begins reducing (Not stopping!) the pressure rise rate.

 

If you're converting to discs at the same time, then use the WJ prop valve too.  But with drums, it'll probably be sending too much pressure to the rear in a panic stop...   Might be OK if you run loaded mostly, or on a long box if you're not afraid to "drive" when the rear starts coming around...

 

With very little experimentation, it would be possible to "tune" an XJ (or any) prop valve to have the knee at a desireable place, but my guess is that for a light to no-load condition, the XJ valve is probably in the ball park.  You just need to make sure your rear brakes are "tuned up" so they actually work.  The "self" adjusters rarely self adjust, unless you use the parking brake a LOT, or back up and stab the brakes a LOT.  :) 

 

Unless you're only driving the truck for one load condition, and one road condition, this valve with one "IN" and one "OUT" is NOT the valve you want, nor is anything like it:  P/N 884-A0707

This valve only blocks pressure to the rear from going higher than a set point, which means if you tune it with an empty truck, and then load your truck, you got WAY less rear brake than you need...  As a former brake system engineer, I do NOT recommend this route.  A regular proportioning valve would be much better for almost all of us!  :)

[1stDeuce]

Update: I found the dissected XJ proportioning valve. And, I'm ashamed to admit, it does NOT appear to work the way I thought it did. In fact, after studying it more closely, I find it's a bit more sophisticated than I thought, and ... I have no clue how it works at all under normal braking. (Which may be why the hot tip in NAXJA was to remove the cup washer and disable the dang thing).

 

I'll post photos in a day or two.

Thanks Eagle... I'd look online and find a diagram, but I really need to get back to work on my MJ bed.   I'm pretty confident that you'll find that the XJ prop valve works like I've explained it...  (I was a brake system enginerd for TRW for 7 years before I got the heck outta Detroit.  And I've stayed at a Holiday Inn Express a LOT!)   :)    

 

As I said before, the usual problem with drum brake XJ's and TJ's is that the self-adjusters don't work for crap, and that leads to the rear brakes not working for crap...  Either that or the axle seals leak 90wt onto the brakes and render them useless.   But all prop valves work to slow pressure rise at the rear above a set pressure level.   If I get some time, I'll find a diagram and post it up.

 

Actually, forget me doing it, here's a link to an excellent explanation on Grassroots Motorsports forum of how a factory prop valve works:    https://grassrootsmotorsports.com/forum/grm/learn-me-adjustable-proportioning-valves/63619/page1/    The 4th post, by Warren, is the one that shows proper function.  The first graph shows how the knee point (Labeled Change-over pressure) tries to approximate the curve that's needed due to weight transfer.   The second chart shows how the SSBC and Jegs adjustable distribution valves above work.  The dial allows you to choose which knee point or change-over pressure you want.  

 

Note that this is NOT how the one inlet/one outlet "prop valve" that many people install works.  For that valve, the knee goes flat, meaning up to a certain pressure, rear pressure is unchanged from what's coming out of the master cylinder, but once you hit a set pressure, no more pressure will go to the rear no matter how hard you stomp on the pedal.  You can see why I'm NOT a fan of this kind of "prop valve", which is really just a pressure regulator.    (Most people do NOT understand this, and it certainly isn't advertised... 

 

Aren't brakes fun??  :)

[knever3]

This thread is fantastic and should be a sticky!  I seem to have a plugged block and it looks like I am going with the JEGS 555-63025 adjustable valve and be done.  It looks like it has the same thread pitch 3/16" lines so hopefully I won't have to remake my hard lines.  Either way it seems like a much better route then simply deleting the load sensing valve.  Actually the OEM got it right, but with lifts and rotted parts it's not easy adapting the new changes.

[Eagle]

On 11/28/2021 at 10:49 AM, knever3 said:

This thread is fantastic and should be a sticky!  I seem to have a plugged block and it looks like I am going with the JEGS 555-63025 adjustable valve and be done.  It looks like it has the same thread pitch 3/16" lines so hopefully I won't have to remake my hard lines.  Either way it seems like a much better route then simply deleting the load sensing valve.  Actually the OEM got it right, but with lifts and rotted parts it's not easy adapting the new changes.

 

That's a good choice, BUT ... pay attention to this from the instructions:

 

Quote

The switch supplied with the Prop Blocks a pressure switch designed to activate the brake lights on the vehicle (and not the brake warning light on your dash). If your vehicle is equipped with  a brake light switch on the brake pedal, the switch in the Prop Block will not be used.

 

By using this proportioning valve, you will lose the warning light that's intended to signal the loss of the front or rear brake circuit. Technically, I think that may be illegal -- but I think most of us know the difference between a firm pedal and the low pedal that results when you lose half your brakes, so I would be concerned about this only if you live in a place that requires annual safety inspections and the inspection verifies that the warning light is present and functional.

 

And this unit is also a work-around if you've swapped in a different brake pedal setup from a different year, and the mechanical switch isn't getting the job done. This allows you to remove the mechanical switch in the pedal assembly and just extend the two wires through the firewall to the pressure switch on the proportioning valve.

 

Also note that two of your hard lines use a 1/2-20 flare nut, and this unit uses all 3/8-16 flare nuts. So you will have to re-make or adapt two of your lines.

[knever3]

Well, that idea just tanked in my opinion, thanks for the direction again Eagle.  I guess I will just buy a new one or clean the one I have.

 

[Eagle]

I'm not sure why what I wrote cancels out the aftermarket adjustable proportioning valve but, if that's a deal-breaker for you, so be it. I don't think you can buy a new distribution block for the MJ, and it's unique -- totally unlike the XJ/ZJ/TJ proportioning valve. If you are going to open it up and clean it out, consider opening up the ID on the ports. Cruiser has made some measurements, which should be posted around here somewhere, which showed that for some reason the orifices on the MJ distro block are smaller than on the XJ/ZJ proportioning valve. IMHO it can't hurt to open them up slightly to match the size of the orifices in the XJ/ZJ units.

 

[CMRCTRY]

Eagle or others that have knowledge.

 

Is the below a viable configuration for adapting to a Ford 8.8 with disk and duel diaphragm upgrade?

 

"The simple way to eliminate the rear height sensing valve is to disconnect the rear axle flex hose from it, run a new hard line from the "nose" of the front distribution block directly to the rear flex hose, and to plug the forward/lower output on the distribution block. That's it -- done."

 

Or

WJ valve with the intent of upgrading to full WJ front brakes in the future?

 

Thanks,

[Eagle]

7 hours ago, CMRCTRY said:

Eagle or others that have knowledge.

 

Is the below a viable configuration for adapting to a Ford 8.8 with disk and duel diaphragm upgrade?

 

"The simple way to eliminate the rear height sensing valve is to disconnect the rear axle flex hose from it, run a new hard line from the "nose" of the front distribution block directly to the rear flex hose, and to plug the forward/lower output on the distribution block. That's it -- done."

 

Or

WJ valve with the intent of upgrading to full WJ front brakes in the future?

 

Thanks,

 

The first option above - just eliminating the rear height sensing valve - will leave you with 100% full braking power to the rear wheels at all times. Since normally about 60% of the weight is on the front wheels, and that increases when you brake, having full power to the rear brakes at all times often results in premature lockup even with the factory drum brakes. With rear disk brakes I would expect it to be worse.

 

The problem with premature rear lockup is that it usually causes a spin-out.

 

The WJ proportioning valve is a proportioning valve, designed and intended to reduce premature rear wheel lockup. If you are going to rear disks, IMHO you definitely need some kind of proportioning valve. Whether the WJ unit is the best choice, I don't know.