Proportioning Valves

[Shoprat007]

Hello out there, just signed up.

I had the same question about which line was pressure. Have my answer, my lines are correct.Great information, thks.

[1stDeuce]

 

Thanks for the great pictures Eagle!!

 

It looks like the XJ prop valve isn't a pressure reducing valve after all...  It looks to be a pressure limiting valve...   Unless that spring loaded piston is more than one piece, it basically only allows pressure to build to a point, then no more.  (I'll take one apart and confirm as soon as I get one...)  It will allow full pressure to pass up to a "knee" or cutoff, after which you'll get no additional pressure to the rear brakes.  

 

On a pressure reducing valve, the rear pressure output is at the end of the screw-in part, instead of below it, and there is a multi-piece/multi spring piston assembly inside that actually allows pressure in the in the rear circuit to continue to rise after the "knee" is reached, but at a reduced rate.  That's the better style valve for performance use, but for a Jeep, evidently limiting the pressure was felt to be enough.   (Which means the prop valve function is probably only having an effect at REALLY high pressures in an XJ, where weight transfer off the rear axle is significant...)

 

I still can't find anything that says how the Wilwood single in/single out adjustable valve works, but because of the design, I do believe it's a pressure limiting valve instead of pressure reducing.   Note that even on the OE valve, you can raise the knee point by shimming the spring, or using a stiffer spring.  The knob on the Wilwood unit simply adjusts the preload on the spring, thus raising or lowering the knee point.  

 

Here's a quickly drawin graph showing the difference between pressure limiting and pressure reducing prop valves, and how they relate to the ideal pressure distribution that they're trying to approximate.

 

 

I'll update after I dissect an XJ valve...   

 

And I should also add that the OE load sensing prop valve on our trucks is the best thing for them if the load is varying...  Unfortunately, the complexity of the 30yo system is less than ideal at this point, and usually a standard prop valve will do the job well enough. 

[A-man930]

Back from the dead!

 

I've added some labels; do I understand the factory MJ distribution block correctly?

 

[Pete M]

found a pic I did years ago.  note the "alteration" I had to do as I did this pretty much blindly :D (internet just wasn't the same back then). 

 

LONG LIVE THE INTERNET!!!!   :bowdown:    :bowdown: 

 

[Rockfrog]

Yeah, in the cutaway ... The RT front comes out through the shuttle valve cap.

 

And if anyone is still curious how the XJ prop valve works ...

There is no emergency bypass circuit as it was labelled back when this thread started. That is the rear circuit itself. Each braking circuit is fully independent of the other (as is the MJ systems). Input passes through the shuttle and into the propotioning valve/spring. The valve is a simple brass seat valve that prevents application of the rear brakes until enough pressure is achieved to actuate the valve (which is set by the springs rate). At which time braking pressure is passed through to the rear circuit. The rubber oring seal is simply there to keep fluid out of the spring chamber. The sealing is done at the tapered end of the valve body (to the left of the rubber seal). The earlier GM and Chevy valves used to retain pressure behind the rubber seal and so had to be disabled to allow pass through bleeding. Modern valves limit application until that pressure is achieved (essentially working backwards).

 

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[A-man930]

Got it.  I'm pretty well sold on retaining the stock MJ dist. block if for no other reason than to keep a brake warning light, but I might even entertain retaining the emergency bypass circuit feature...  Any reason not to do this? 

 

I do have an adjustable prop. valve like this one waiting for the overhaul.  

 

The stock block, the adj. proportioning valve, and a 10psi residual pressure valve in the primary line to the rear should aside from the automatic adjust-ability of the height sensing valve be an upgrade, no?  

[Pete M]

if I had to do it again, I'd either swap in an XJ/ZJ/whatever unit or splice both rear MJ ports into a single line and then run that to the back.  just in case loss of front brakes shuts off the normal rear port as it opens the emergency port.  unless someone can ease my mind about that.

[Eagle]

Yeah, in the cutaway ... The RT front comes out through the shuttle valve cap.

 

And if anyone is still curious how the XJ prop valve works ...

There is no emergency bypass circuit as it was labelled back when this thread started. That is the rear circuit itself. Each braking circuit is fully independent of the other (as is the MJ systems). Input passes through the shuttle and into the propotioning valve/spring. The valve is a simple brass seat valve that prevents application of the rear brakes until enough pressure is achieved to actuate the valve (which is set by the springs rate). At which time braking pressure is passed through to the rear circuit. The rubber oring seal is simply there to keep fluid out of the spring chamber. The sealing is done at the tapered end of the valve body (to the left of the rubber seal). The earlier GM and Chevy valves used to retain pressure behind the rubber seal and so had to be disabled to allow pass through bleeding. Modern valves limit application until that pressure is achieved (essentially working backwards).

 

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Incorrect.

 

The normal fluid route in the XJ valve enters from the top right (in my photo of the XJ valve). This port is to the right of the cup washer on the plunger, directly into what you refer to as the spring chamber. From there, the fluid departs by going around the cup washer and then turning 90 degrees down, to the outlet port.

 

The port that I circled and labeled "Emergency Bypass Circuit" is exactly that. It is normally blocked by the right-most O-ring on the shuttle valve. If the front circuit fails, pressure to the rear pushes that shuttle valve to the left, and wnen the O-ring has cleared the emergency port the emergency bypass circuit is fully open. There certainly IS an emergency bypass circuit in the XJ valve.

[Rockfrog]

From my understanding, the MJ's emergency circuit simply bypasses the LSV (prop valve) in the event of front brake failure, supplying the rears with full brake pressure regardless until the condition is resolved.

 

The XJ prop valve does not need this, due to its design. In the event of a front circuit failure pressure will build, open the prop valve and full pressure to the rear is achieved.

 

FYI - the MJ and XJ share the same brake warning switch and shuttle design. Slap an XJ prop valve in there and plug in the switch plug and you'll still have a warning light that will function just as it always did. Wanna guess how I know?

 

 

 

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[Rockfrog]

Yeah, I see it, was looking at it with crossed eyes again.

I stand corrected.

 

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[A-man930]

2 more questions:

 

-does anybody know if the seals in the MJ valve are replaceable and where to get them?

 

-And again on the MJ valve, was there not a concern that the emergency bypass ports aren't drilled correctly?  I thought I remember one of the cutaways showing an incomplete passage...

[Eagle]

Yes, on the MJ distribution block I cut open the bypass port was not drilled all the way up to intersect the main fluid passage. If the front brakes had failed on that truck, there would not have been a by-pass circuit.

 

But that's only one sample. I don't have any other MJ blocks that I'm willing to sacrifice to find out whether or not the hole is drilled all the way.

 

It's the same unit that's shown in the first photo of my opening post in this thread, but this time I edited the photo to show the passage the way it's supposed to be drilled. It's a straight passage, so you could check it by removing the shuttle valve (the horizontal plunger across the top) and probing the bypass orifice with a piece of fine wire.

[A-man930]

Yes, on the MJ distribution block I cut open the bypass port was not drilled all the way up to intersect the main fluid passage. If the front brakes had failed on that truck, there would not have been a by-pass circuit.

 

But that's only one sample. I don't have any other MJ blocks that I'm willing to sacrifice to find out whether or not the hole is drilled all the way.

 

It's the same unit that's shown in the first photo of my opening post in this thread, but this time I edited the photo to show the passage the way it's supposed to be drilled. It's a straight passage, so you could check it by removing the shuttle valve (the horizontal plunger across the top) and probing the bypass orifice with a piece of fine wire.

Gotcha.  I was going to try and squirt some brake clean through it with the valve out.