Proportioning Valves

[Eagle]

The stars finally aligned, my camera batteries weren't dead, and my Internet connection was functioning. I found the XJ proportioning valve I had sliced and diced, so I can now post photos to show how the XJ proportioning valve is different internally from the MJ front distribution block.

 

First up, the MJ distribution block. I've posted a photo of this one before -- this photo is touched up to show how the emergency bypass circuit is supposed to run:

 

 

 

And now the XJ proportioning valve. Notice the plunger, spring, and rubber cup washer. That's the proportioning mechanism, but after studying it I can't figure out how it works. If you look closely, you'll see that the outer surface of the rubber cup washer has ribs, so perhaps the ribs allow brake flow until the pressure gets high enough to compress the ribs and allow the cup to seal against the bore? Dunno.

 

I also realized that the XJ valve has a bypass, similar to the MJ, so if the front brakes fail the rears will get full flow and pressure, not proportioned.

 

[HOrnbrod]

And now the XJ proportioning valve. Notice the plunger, spring, and rubber cup washer. That's the proportioning mechanism, but after studying it I can't figure out how it works.

 

You're not the only one.  :hmm:

[shelbyluvv]

http://fsjnetwork.com/forum/viewtopic.php?t=1292

 

When there is more fluid pressure to the front then the rear it will shuttle the valve to the open position and bypass the rear circuit. There are two different GM prop valves like the XJ one.

 

One you have to pull the plunger out to bleed the rear circuit. There is a special tool that holds it open.

 

The other style pushes the plunger out to bypass the rear circuit. You have to hold the plunger in to bleed the rear circuit. There is a different tool needed for that style.

 

I learned this fighting with a J10 that had no rear brake pressure.

[HOrnbrod]

Thanks Bo. That makes perfect sense.  So, do XJs need these special tools to bleed them? Had several but never had to bleed the brakes on one. I could check my FSM but I'm lazy tonight.

[Eagle]

http://fsjnetwork.com/forum/viewtopic.php?t=1292

 

When there is more fluid pressure to the front then the rear it will shuttle the valve to the open position and bypass the rear circuit. There are two different GM prop valves like the XJ one.

 

One you have to pull the plunger out to bleed the rear circuit. There is a special tool that holds it open.

That's the horizontal plunger across the top. Both the XJ and MJ units have that. It's cleverly called the shuttle valve, and it also actuates the brake system warning light. You can see how the plunger for the switch is down in the narrowed "neck" of the shuttle plunger. If the valve moves in either direction (caused by a failure of either the front or rear cirsuit), it'll push the plunger up and turn on the light.

 

It's how that lower, proportioning plunger in the XJ valve works that has me puzzled.

[Eagle]

Thanks Bo. That makes perfect sense.  So, do XJs need these special tools to bleed them? Had several but never had to bleed the brakes on one. I could check my FSM but I'm lazy tonight.

XJs don't need any special tools. Bleed normally (start at right rear, progress to left front last). If the shuttle valve hasn't reset to cancel the warning light, stomp on the peddle a couple of times to reset it.

[shelbyluvv]

Check this out Eagle.

 

[Eagle]

Interesting video, but IMHO useless. The XJ proportioning (or "combination") valve and the MJ distribution block don't have what he referred to as a "metering valve" in the rear of the block. His explanation of how the proportioning spring works doesn't at all correspond to how it seems to work based on what we can see by cutting one open.

 

His explanation of the shuttle valve operation is backwards.

 

I'd like to see an example of the valve in that video sliced open to show how it works.

[Eagle]

Here's a closer view of just the proportioning valve segment of the XJ valve. The upper vertical passage to the left is a full-pressure by-pass for the rear brakes in case the fronts fail. Under normal conditions, the shuttle valve blocks it off (as shown). If the front circuit loses pressure, the shuttle valve moves to the left, opening the straight-through path to the bottom output passage.

 

 

That much is easy to figure out. What's less easy is how it works under normal conditions. The input is the vertical passage at upper right. The fluid (and pressure) come down that passage and into the chamber the right of the cup washer on the piston. Then somehow the fluid and pressure have to get past that cup washer and into the lower vertical outlet passage at the lower left.

 

I don't get it.

[Pete M]

[Eagle]

Going back a decade, the "hot" trick in NAXJA was to just remove the cup washer and spring, and leave the plunger/piston in the forward position (pulled into the cap nut, leaving the orifice open). I never heard of anyone having a problem doing that.

 

After following Shelbyluvv's link, that led me to another video in which an older gent with a southern accent discussed proportioning valves. He claims they are all just pressure limiting valves. Dunno. It was a long video, at the end of which I knew nothing more about how they actually work. Or if they're really needed.

 

The first car I had with front disc brakes was my first 1968 Javelin. Disc front/drum rear, and it did not have a proportioning valve. However, the 1968 AMX (which was the shorter, 2-seat version of the Javelin) did use a rear proportioning valve. Shorter wheelbase ==> more tendency for weight transfer to the front, thus greater tendency to spin out under heavy braking.

 

One of my friends bought a used AMX. Drove it home a few miles from where he bought it, and all the brakes locked up and burned up the linings. Turned out the previous owner had used ATF in the brake system, and the ATF caused all the seals to swell up. Most of the parts were available or rebuildable. What we could not find (or figure out how to rebuild) was the rear proportioning valve. So we just eliminated it. We knew what it did, so we took an alternate approach to reducing the braking at the rear wheels. His AMX was a 343 c.i.d. V8. We used wheel cylinders from a 6-cylinder Gremlin. They were smaller in diameter, and we calculated that would reduce the rear braking power by about 25 percent. He drove that car for twenty years or more like that, including autocrossing it, and never had a problem.

 

I wish there was a new manufacture ( or quality rebuilt) source for the MJ height sensing valve, because they way they operate makes a lot more sense to me than the black magic of the XJ combination valve.

[A-man930]

Eagle, can you post up the original image (with your notes added in) of the MJ valve?

[Eagle]

Eagle, can you post up the original image (with your notes added in) of the MJ valve?

The one I posted awhile back didn't have any notes. It was a different photo of the same MJ distribution block that's shown in the new photo.

[A-man930]

I see.  Then could you (or anyone for that matter) label the inlet/outlet/etc. ports on the MJ valve or link to the picture I know I've seen where they're labeled... ?

[HOrnbrod]

[ScottJeep]

I've been reading an attempting to absorb as much as possible..

( not easy normally plus a day of helping those much worse than me at work and I might as well be drunk ( support engineer for everything you can imagine)). 

 

After fully rebuilding my new to me Dana 35 (for the 4.11 ratio) complete brakes..new shoes, springs, cylinders, drums checked resurfaced.new parking brake cables. ..new bearings seals.....

(it's pretty)

 

The idea was a one day swap. Was successful but after bleeding... I use a Motive bleeding unit since I have to do everything by myself or

it doesn't get done.... 

 

ANYWAY....

 

Have brakes but MUCH less than before. 

 

Now I have not gone back and adjust more but man I was hoping for something before moving to my WJ front brake fun. 

 

My only thought is the rear LSV. There is zero resistance if you attempt to spin the arm either way. 

[Eagle]

Did you follow the procedure for bleeding the bypass circuit, or did you just do a normal bleed at each wheel?

 

Is the arm of your load-sensing valve horizontal?

[HOrnbrod]

You won't feel any resistance when rotating the LSV arm. The arm position based on the vehicle height varies the valve's internal fluid port size to change the bias to the rear brakes. If should rotate freely 180* (EDIT: 360*). If it doesn't there's something wrong. Did you snug up your brake shoes so they just rub on the drums when rotating the wheels?

[ScottJeep]

Eagle, 

 

I did not do anything other than a normal bleeding... 

 

Will search next for the bypass procedure that I am guessing is here someplace....apologies

 

The arm is "slightly" non-horizontal....slightly towards the bed. 

 

Don, 

 

That thing rotates....360 ( did I break it?) without any "force" to make do so at all. 

 

Set the shoes just slightly dragging.  

 

Since they were new, expected that after a couple of days use and using the new parking brake cables they would need another adjust. 

[HOrnbrod]

That thing rotates....360 ( did I break it?) without any "force" to make do so at all.

 

My typo - I meant 360*.  :doh:   Yes, that's normal.

[Eagle]

The arm is "slightly" non-horizontal....slightly towards the bed.

Try pointing it slightly below horizontal.

 

Think how it works. Normal (unloaded) the arm should be horizontal. There's a rigid rod from the end of the arm up to the chassis. Put weight in the bed. More weight needs more brakes. Bed goes down, rod goes down, axle stays at same height ==> arm goes down.

[HOrnbrod]

 

The arm is "slightly" non-horizontal....slightly towards the bed.

Try pointing it slightly below horizontal.

 

Think how it works. Normal (unloaded) the arm should be horizontal. There's a rigid rod from the end of the arm up to the chassis. Put weight in the bed. More weight needs more brakes. Bed goes down, rod goes down, axle stays at same height ==> arm goes down.

 

I don't think so. More weight, bed goes down closer to the axle. So the stationary rod moves the valve arm up (not down) above horizontal to the axle to increase rear brake bias.

 

So try pointing it slightly above horizontal.

[Eagle]

 

 

The arm is "slightly" non-horizontal....slightly towards the bed.

Try pointing it slightly below horizontal.

 

Think how it works. Normal (unloaded) the arm should be horizontal. There's a rigid rod from the end of the arm up to the chassis. Put weight in the bed. More weight needs more brakes. Bed goes down, rod goes down, axle stays at same height ==> arm goes down.

 

 

I don't think so. More weight, bed goes down closer to the axle. So the stationary rod moves the valve arm up (not down) above horizontal to the axle to increase rear brake bias.

 

So try pointing it slightly above horizontal.

 

:doh:

 

Don is correct. I was picturing the valve as mounted on the axle, not on the frame.

 

Pay no attention to that man behind the curtain ...

[shelbyluvv]

I overloaded the MJ last week. When I finally got all the weight out of the bed, any minor taps of the brakes would lock up the rears. I had to disconnect the rod and panic stop the truck to relieve the pressure in the LSV. She works fine now.

 

Be careful with adjusting that angle. It doesn't take much to lock up the rear on these light truck.

 

[ScottJeep]

OK... pretty much ended up bathed in brake fluid but in the end... brakes! 

 

Still need to fully re-adjust the rear shoes.

 

AND

 

Thinking of just making a new bracket for the LSV arm or drilling a new hole, countersinking the bolt head and calling it done. 

 

Now to the prepping for the WJ Akebono front brakes and all that fun. 

 

Thanks for the help!!