Brake porpotioning valve...

We've covered this before?

Ohwell.

I'm looking to remove it, as it never functioned for me anyways and all my brake lines back there are rather on the rusted through side. I'd also like to run my brake line from the center of the crossmember down to the pumpkin.

So, it has a return line from it, which and it's seperate from the line that actually runs to my axle. From this I can assume that it allows pressure to bleed off the line and back to the MC. Am I right? Or do I have it all wrong? My light broke in my garage, so I was kinda guessing.

Now, to remove it. All I have to do is remove the T that runs to it, pull it off, and plug the return line at the MC? (It's all an assumption that the return line goes to the MC, I couldn't see... stupid value village lightbulbs)

Or am I off my rocker?

Sort of. What we all collectively referred to as a "return" line is actually a safety line. It is pressurized via the prop valve under the MC in the event of a front brake failure...it sends full braking to the rears. (credits go to Eagle for digging thru the FSM and finding this info out).

I, and a few others, have removed the rear valve and eliminated the second line, perhaps unwisely :oops: Everything functions normally, BUT in the event of a front system failure (which I've never experienced) I suspect the system would be compromised. A better way to replumb the whole mess IMHO would be to keep the second line, eliminate the height valve in the back, but tee the two rear lines together. That way you still retain the emergency circuitry that's <> built in. Or you can swap in an XJ prop valve that doesn't have the safety circuit.

FWIW, the second line exits the prop valve from the bottom, in the front. It's not the one that comes straight out of the valve. I capped it off @ the valve body with a 3/16" plug, or something close to that.

Jeff

Safety isn't an issue as I see it. I'm going to die young one way or another, so it might as well be in my jeep.

Okay, so I should just get the flare wrenches out and have at it, I think. Although, I guess I should hit up napa again first for some more brake lines.

Like I said, it works but isn't the ideal way to do it.....caveat empetor, or something like that :)

And remember, it doesn't have to be pretty, it just has to work. I find that 3/16" unions (compression fittings) are your best friend, unless you're into flaring.....

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Jeff

I'm not into flaring ;)

I got rid of my rear bias valve, just running a single line from the front proportioning valve to the rear rubber brake line. I used a XJ proportioning valve rather than block off the 2nd rear line. Why? Because I had one on hand.

Running an XJ prop. valve, or capping the 2nd line on the MJ prop. valve, are both safe. It basically makes it the same setup as the XJ. All that second line is good for is adding fluid for the rear bias valve.

I'm not into flaring ;)

 

I got rid of my rear bias valve, just running a single line from the front proportioning valve to the rear rubber brake line. I used a XJ proportioning valve rather than block off the 2nd rear line. Why? Because I had one on hand.

 

Running an XJ prop. valve, or capping the 2nd line on the MJ prop. valve, are both safe. It basically makes it the same setup as the XJ. All that second line is good for is adding fluid for the rear bias valve.

This is exactly what I have done as well. It works perfectly and the brakes are excellent. If I brake normally, the Jeeps stops as expected. If I stand on the brakes, the back will lock up first. I ran new line complete front to back. I used a 96 Dakota hose for the extra legnth it offered.

CW

We *may* have some new info coming forth from Eagle about how the front prop works. He's having one cut in half to see how the insides are plumbed. I'll keep everyone posted.

Jeep on!

--Pete

I always thought brakes were one of those fancy options you pay extra for :twisted: I have been having severe brake fade, especially when I am going up a steep inlcine, and need to hold the brakes, and give gas at the same time, Or when I have to back down something to get a better line. I am thinking that I am going to be "fixing" my brake system before long.

Patrick,

This brake fade could be mostly your pads.

I did what I did for a couple reasons, my lines where shot, all the way to the T case. I couldnt see splicing in so I ran completely new lines. I needed a longer rear hose anyway and the complicated bellding method riquired for the rear valve was ...well stupid. The rear brakes just sucked!! I was moving some furnature for my sister just before I did the rear brakes. She moved to a very hilly portion of the state. I had a great difficulty stoping on hills and by a couple trips my front brakes where glazed so badly I almost couldnt stop. I was supposed to get better brakes with the extra weight in the rear, did'nt happen. I had to replace my front pads when the move was done, this should never have happened. Tghats when I started really looking at the rear braking system. I do not proclaim to be smarter than the engineers who designed the system, but I gotta say, there must be a reason why no one else uses it!!

Everything is fine now, I have moved my bed full of mulch twice now. Quite a heavy load, more than I should have at one time. based on the 'sag" of the rear springs whan full. NO PROBLEMS!!!

I spoke with eagle acouple times before and during the rebuild. He is a very knoledgeable individule.

Good luck,

CW

Well, all I know is I need to do something. I was weven thinking of doing a line lock or something similar :shock: All I know is My brakes are in "good" condition, and then don't hold me on a steep incline cvery well.

Patrick

I used the original '78 pads for a while after the axle swap (they still had plenty of thickness left and I was broke). They stopped the truck okay enough on the road, but couldn't hold the truck on inclines no matter how much I stood on the pedal. I installed new pads and everything works great now.

Jeep on!

--Pete

I guess I might try replacing all my brake pads.. it really can't hurt.

Patrick

We *may* have some new info coming forth from Eagle about how the front prop works. He's having one cut in half to see how the insides are plumbed. I'll keep everyone posted.

Jeep on!

--Pete

Taking that as my cue, here's what I found:

The front "combination" valve, on an MJ is NOT a combination valve. There is no proportioning function at all. All proportioning is handled by the load-sensing valve in the rear.

The normal flow to the rear brakes is through the outlet in the "nose" of the combo valve. That path is always available. From the description in the FSM I had conjured up an idea that under loss of the front brakes that circuit was disabled, but such is not the case.

The outlet on the bottom of the front combo valve (the one we have often referred to -- incorrectly -- as a "return" line) is normally blocked off by the plunger in the top of the valve that actuates the brake warning light. If pressure to the front brake circuit is lost, the pluger slides to the rear of the combo valve and opens that port, allowing pressure to flow and bypass the rear combo valve. However, this does not disable the "nose" outlet, so if you eliminate the "return" line completely and run a single line from the nose of the combo valve direct to the rear axle flex hose, you will have full braking power to the rear wheels at all times. As noted, under panic conditions and in slippery conditions, this may result in premature rear wheel lockup, especially if running with no load. (This is why swapping in a Cherokee combo valve is a good idea, if you have access to one.)

Now, here's the scary part: In the one I dissected, the port that is supposed to be the bypass circuit, to provide full braking to the rear if the front circuit fails, was internally blocked. The hole from the bottom, where the outlet fitting goes, wasn't drilled far enough into the valve body to reach the fluid passages. Now I want to get my hands on a few more to check depth of hole, because I have a nagging concern that if one was made defective, many were probably made defective. This won't affect normal braking, but it means that the intended function of the bypass circuit can never be activated. So if you lose the front brakes, the rear brakes will still be fed only through the proportioning valve.

Summary: IMHO, dump the rear proportioning valve. Mine blew up when I made a panic stop. That's not a good omen. IMHO the best solution is to use an XJ combo valve. Next best is to remove the "return" line, plug that bottom outlet, and run the "nose" outlet direct to the rear flex hose. If you find that the rear brakes consistently lock up too soon, get a Wilwood adjustable proportioning valve from Summit Racing and install that in the rear brake circuit.

Did you get to snap a photo?

Jeep on!

--Pete

I also experienced that the rear wheels locked up before the front, when I removed the prop valve.

As a cure until I get an adjustable prop valve, I adjusted the break shoes in the drums inwards (away from the drum), and that worked.

I did several panic-break tests on a snow/ ice covered road, and I can live with the brakes until I install an adjustable valve, and then readjust the brake shoes outwards again.

On winter roads, bad rear brakes are much safer than if they lock up before the front brakes,

I did also consider to apply some oil on the shoes, in order to reduce the rear brake lock up, but it wasn’t necessary.

I can imagine you guys running bigger tires, 33”-35” and bigger, have weak brakes.

You haven’t considered hydroboost brakes?

I did install hydroboost on my YJ after I put on 35”, and the brakes are extreme.

I’m able to lock up the wheels in 60mph on dry asphalt, I’m not saying that’s the correct way to stop the Jeep in 60 mph, but I can if I have to, before the hydroboost it was impossible.

you have any more info on that setup? I'm not familiar with how it works at all. I assume it's just like a secondary brake booster? I need to change out my brake booster, so I was just going to go with a 96+ xj dual diaphragm model.

Hydroboost uses the PS pump instead of engine vacuum. The benefit is that it can create absurdly high pressures, whereas the stock (any year) vacuum brake booster can only make upwards of ~1200psi (I think, don't quote me). With the vacuum assist booster, it doesn't matter how hard you press the pedal, that's all the pressure it can make. The hydroboost can do 2000+psi without flinching. Another benefit of hydroboost is it will still function if you have an absurdly huge cam, but don't ask me why you'd have that in a 4x4.

Don't quote me on the numbers, it's something a read a long time ago.

good to know.

It's not something I ever plan on doing, but I'm curious about everything. I'm pretty sure I'm gonna put in a 96+ booster, a stock XJ propo valve and new lines.

DirtyComanche is correct, the setup is quite simple, the only thing is that your stock PS pump have to be upgraded or replaced with a 1500 lbs pump and (the stock PS pump is 1100lbs).

On the YJ I did install an additional pump to supply the hydroboost unit and the MM hydraulic winch, used a GM pump from a Chevy 4x4 pickup.

Did weld a pump bracket and installed the pump where the AC compr. suppose to be.

You can check out this page for compl. hydroboost units

http://www.vancopbs.com/category_s/66.htm

A lot of Chevy/Ford/Dodge trucks have stock hydroboost, so finding parts on the junkyard should be easy and then build a cheap setup, that’s what I planning to do when going bigger tires on the MJ.

A have heard several TJ owners that are complaining they have bad brakes after going 35”, although all TJ’s have a double membrane vacuum unit as stock, think it’s the same as + 96 XJ’s.

I'm pulling another post out of the files. Eagle you mention that you were going to check other MJ distribution blocks for the blockage in the bypass outlet. Did you find any? The way your cross section shows it, that vehicle would have a huge air pocket in the bypass line and would never have firm pedal. It could not be bled. If it was made like that, it couldn't have left the factory.

Could someone send me the FSM section pertaining to the rear propotioning valve. I had mine apart and I suspect the function is a bit different than the past posts have indicated. Mine is very rusty. Does the fitting at the top come out? If not, it was a bear to manchine.

What I'm seeing right now is behind the top fitting is a plunger that is pushed against the opening to close the port. The plunger is held against the opening with a spring that is adjusted by the position of the lever arm. This spring and the small diameter plunger regulates the pressure of the rear lines and responds to accordingly to more right foot forces, but always less than the front pressure with light loads. This would mean the full time pressure would be available at the top fitting, not the side one.

I'm confused on the second line. I'm thinking when there is a front brake failure full pressure is available to the side inlet of the proportioning valve and forces the plunger up and closes the inlet, thus allowing full pressure to the drums. The problem is how is this line bled. This line could only be bled by causing a pressure imbalance, front to rear (as stated in FSM?) and cracking the fitting at the prop valve.

The function as described in previous post would result in slow (on and off)rear brakes by metering the fluid through a restriction, which is not what is desired.

Perhaps if I could read the FSM, I could confirm the function. It definetly looks like I'll be removing the prop valve, but I would like to know how it works first.

512tr

You have to tell me more about that engine, I love it :D

Charles

I'm pulling another post out of the files. Eagle you mention that you were going to check other MJ distribution blocks for the blockage in the bypass outlet. Did you find any? The way your cross section shows it, that vehicle would have a huge air pocket in the bypass line and would never have firm pedal. It could not be bled. If it was made like that, it couldn't have left the factory.

Could someone send me the FSM section pertaining to the rear propotioning valve. I had mine apart and I suspect the function is a bit different than the past posts have indicated. Mine is very rusty. Does the fitting at the top come out? If not, it was a bear to manchine.

What I'm seeing right now is behind the top fitting is a plunger that is pushed against the opening to close the port. The plunger is held against the opening with a spring that is adjusted by the position of the lever arm. This spring and the small diameter plunger regulates the pressure of the rear lines and responds to accordingly to more right foot forces, but always less than the front pressure with light loads. This would mean the full time pressure would be available at the top fitting, not the side one.

I'm confused on the second line. I'm thinking when there is a front brake failure full pressure is available to the side inlet of the proportioning valve and forces the plunger up and closes the inlet, thus allowing full pressure to the drums. The problem is how is this line bled. This line could only be bled by causing a pressure imbalance, front to rear (as stated in FSM?) and cracking the fitting at the prop valve.

The function as described in previous post would result in slow (on and off)rear brakes by metering the fluid through a restriction, which is not what is desired.

Perhaps if I could read the FSM, I could confirm the function. It definetly looks like I'll be removing the prop valve, but I would like to know how it works first.

looks for a post done by Eagle on the subject, he had one cross sectioned.

I'm pulling another post out of the files. Eagle you mention that you were going to check other MJ distribution blocks for the blockage in the bypass outlet. Did you find any? The way your cross section shows it, that vehicle would have a huge air pocket in the bypass line and would never have firm pedal. It could not be bled. If it was made like that, it couldn't have left the factory.

It did leave the factory. That was a take-out from an MJ ... the owner replaced it with an XJ combo valve.

No, I haven't sliced another to see if it has the same problem. I only have one more, and I'm not willing to cut it apart. I suppose I should just probe and see how deep the hole goes.

What I'm seeing right now is behind the top fitting is a plunger that is pushed against the opening to close the port. The plunger is held against the opening with a spring that is adjusted by the position of the lever arm. This spring and the small diameter plunger regulates the pressure of the rear lines and responds to accordingly to more right foot forces, but always less than the front pressure with light loads. This would mean the full time pressure would be available at the top fitting, not the side one.

I'm confused on the second line. I'm thinking when there is a front brake failure full pressure is available to the side inlet of the proportioning valve and forces the plunger up and closes the inlet, thus allowing full pressure to the drums. The problem is how is this line bled. This line could only be bled by causing a pressure imbalance, front to rear (as stated in FSM?) and cracking the fitting at the prop valve.

I recently posted the text from the FSM on bleeding the MJ brakes. The second line is a by-pass line that is active only if the front brake circuit loses pressure. In that case, the slider in the top of the front combo valve moves and allows fluid to flow through the by-pass circuit (which, of course, was blocked in the MJ front combo valve I sliced in half). Bleeding requires opening a bleed screw on a front caliper to simulate a front brake failure, allowing the by-pass circuit to be bled. Then you close the bleeder and bleed all four wheels normally.