definitive comanche brake bleeding

[badguru]

Ok let's start from the top. When ur jeep was being built it came down an assembly line. At sum point a poor slob had about 2 minutes to make the final connections of the brake lines, fill the system, and bleed them. All in 2 minutes. So it can't be all that hard right well it's not hard at all. Lets look at the system. Master cyl has 2 lines coming out and going to the proportioning valve right? Right okay next the prop valve has only 3 lines coming out of it that's right only 3. 2 For the front calipers and 1 for the rear drums that's right only 1 for the rears. So what's that other line. That everyone likes to put a plug into?

Its a (drumroll please)

RETURN LINE.

Why would u have 2 lines going to the same place?

So after bleeding ur rears move up to the prop valve (big nut lower front) pump up the brakes like ur doing a wheel cyl and just crack the nut enough to let the air out. Repeat till fluid is running with no air. Please don't tell me that might leave air in the system because believe it or not (trademark Ripleys) you always have sum air in you brakes. Oxygen loves iron that's how ur truck got rusty. Thus ends today's lesson.

[Eagle]

I'm sure the above is intended to be helpful, but unfortunately it's completely wrong.

 

The thing in the front that you refer to as a proportioning valve is not -- in the MJ that distribution block has NO proportioning function. The one in the XJ is a proportioning valve, but not in the MJ. I've posted photos of both units sliced in half to show the internals, and it's clear from the photos that the MJ unit has nothing that provides proportioning.

 

The second line to the rear axle in the MJ is not a return line. Brake fluid doesn't circulate -- there's nothing to return, and nowhere to return to. The second line is an emergency bypass circuit. The primary line feeds the rear brakes through the height-sensing proportioning valve in the rear. Unless the truck is heavily loaded, this reduces the amount of braking to the rear wheels. If the front circuit fails and all you have is the rear, you want full power to the rear, so the second line is an emergency circuit that bypasses the rear proportioning valve and sends full power to the rear brakes.

 

Here's the XJ proportioning valve:

 

 

And here's the MJ distribution block:

 

Your bleeding suggestion does not agree with the factory service manual, and will not bleed air out of the emergency line. Since air in the emergency line would result in not having functional rear brakes when you need them most (if the front brakes have failed), your suggestion is dangerous.

[badguru]

Your front and rear circuits are independent of each other so if u lose ur front you still have a sealed working rear circuit that will stop your truck. So why would you need an emergency bypass? At the back of your truck where you have that valve. It let's more or less volume of fluid past it depending on it's position. That moves that shuttle or prop valve in the block to give you more or less braking at the rear. The correct term for it is a servo spool just like in a backhoe or a farm tractor you know those oily hand leaver things with all the hoses coming out of them. This RETURN line gives that back pressure at the valve So it can shuttle under pressure to change the amount of volume to the rears. Its a loop system and in wrong to call it a return line. it's a Signal line back to the prop valve. So you have to bleed the air coming from the T-fitting throught the leveling valve back to the prop valve. Same setup on Toyota's and older chevy duallys. I used the same bleeding technic on all 3 of my jeeps and I can lock all 4 Tires and I have a real good petal. But please make a copy from your factory service manual and post it for me to look at. I'm always ready to keep a open mind.

[87MJTIM]

:popcorn:

[Eagle]

Your front and rear circuits are independent of each other so if u lose ur front you still have a sealed working rear circuit that will stop your truck. So why would you need an emergency bypass? At the back of your truck where you have that valve. It let's more or less volume of fluid past it depending on it's position. That moves that shuttle or prop valve in the block to give you more or less braking at the rear. The correct term for it is a servo spool just like in a backhoe or a farm tractor you know those oily hand leaver things with all the hoses coming out of them. This RETURN line gives that back pressure at the valve So it can shuttle under pressure to change the amount of volume to the rears. Its a loop system and in wrong to call it a return line. it's a Signal line back to the prop valve. So you have to bleed the air coming from the T-fitting throught the leveling valve back to the prop valve. Same setup on Toyota's and older chevy duallys. I used the same bleeding technic on all 3 of my jeeps and I can lock all 4 Tires and I have a real good petal. But please make a copy from your factory service manual and post it for me to look at. I'm always ready to keep a open mind.

 

Did you even look at the two photos?

 

The horizontal shuttle valve in the front distribution block has nothing to do with brake proportioning. It serves two functions: First, when tripped in either direction it activates the brake system warning light (by pushing up the plunger in that plastic switch screwed into the top of the body). Second, when the front circuit fails, it opens up a bypass circuit to allow full brake force to the rear wheels. In the Cherokee unit, it does this my allowing fluid to bypass the proportioning plunger in the valve body. In the Comanche, it does it by routing brake fluid through that second line to the rear, which connects downstream of the height-sensing proportioning valve. If it didn't do that, all you would have if the front brakes failed would be the severely reduced rear brakes because the proportioning valve would be restricting the flow. The second line is not a return line nor is it a "signal" line. It's not a loop system -- it's a direct system with a bypass line that is actuated only when the front brake circuit loses pressure.

 

I posted the bleeding procedure from the factory service manual in another thread just last night.

 

I'll post an annotated photo of the MJ distribution block as soon as Photobucket comes back online so I can upload to it.

 

[Edit: Photobucket is back up]

 

MJ distribution block showing the shuttle valve in the tripped position, as it would be if the front brake circuit lost pressure. Compare with the photo above, which has the shuttle in the normal position.

 

[badguru]

That's great if you only drive around with no front brakes. In normal operation that's a signal line that determines how much braking force the rears should have sent to them. Stomping on ur brakes with the front bleeders open to bleed ur rears and having to go around and bleed everything several times to get it bled sound incredible to me. Please post that from ur fsm so I can check it out. Who knows I been wrong before. And I did study the pics closely and everything I see makes me think I'm right.

[fiatslug87]

From the '90 FSM

 

 

[Gene]

Ummmmm....

 

I do not think that procedure is complete. 

 

It will bleed the rear circuit that goes through the height sensing valve.

 

It will NOT bleed the rear bypass "emergency" line.

 

Gene

[Eagle]

That's great if you only drive around with no front brakes. In normal operation that's a signal line that determines how much braking force the rears should have sent to them. Stomping on ur brakes with the front bleeders open to bleed ur rears and having to go around and bleed everything several times to get it bled sound incredible to me. Please post that from ur fsm so I can check it out. Who knows I been wrong before. And I did study the pics closely and everything I see makes me think I'm right.

 

This is SOOOOO incorrect I don't even know where to begin.

 

Brake proportioning in the MJ is handled by the rear height-sensing valve. That shuttle in the front distribution block is NOT in any way involved in proportioning. Look at the photo of the XJ proportioning valve. See the spring and plunger? That's what does the proportioning. The MJ doesn't have any of those parts. Look at the photo of the MJ distribution block in the normal position (the photo with the wicker background). See the O-rings on the shuttle plunger? See where the left-most O-ring is relative to the passage for the emergency circuit? In normal operation, that passage is completely shut off and isolated from the brake system. It only opens if the front brakes lose pressure and the shuttle moves to the right (in the photo orientation).

 

The following is from the '88 MJ FSM:

 

 

After bleeding the brake calipers and cylinders the rear brake by-pass line must also be bled.

 

Open a front caliper bleed fitting and depress the brake pedal to the floor. This will shuttle the by-pass differential valve and allow fluid to flow through the by-pass line. The brake warning light on the instrument panel will illuminate when the ignition key is in the ON position. This signals the shuttling of the valve.

 

Re-Bleed the rear brake cylinders with the front caliper bleed fitting open.

 

After re-bleeding the rear brake cylinders the entire system must be bled again.

 

Bleed the brake calipers and cylinders in the following sequence:

 

1st -- Right rear

 

2nd -- Left rear

 

3rd -- Right front

 

4th -- Left front

[fiatslug87]

Well now you got me thinking. The '89, '90 an '91 FSMs only have the above scanned procedure, but the '92 FSM had this specifically for the MJ, like Eagle's '88. Someone must have over looked it during editing... thanks Chrysler.

[badguru]

My 89 base is an original untouched 83k truck I bought from the original owner and that brake line routing is backwards from the truck I know has original factory lines on it the line coming out the bottom goes to the height sensing valve and from the other side of the hsv to the t fitting. Good luck to anyone using that bled procedure if you have had the whole system drained like I did on my 89 eliminator when I replaced every part of the brake system except the prop valve. Anyway my brakes are bled. I also wish my mj had front leafs like the one in the picture.

[badguru]

Had to see it to believe it. I'm going to open up my front bleeders an see if I have good rear brake. If I do I'm sticking with my procedure. Let you know tomorrow.

[fire797]

The brakes on an MJ have been a point of contention on this forum for a while now. I have read every post on how to up grade them, bleed them and replace them..... This post has been the best so far for confusing me..... I have removed the height sensing valve and the line to it. I have place a plug in the block and I will be bleeding the brakes today or tomorrow..... But I still have not found an answer to one question I have. Why can't I just replace the distribution block on the MJ with a Propositioning valve from and XJ? Won't this give me the safety of having rear brakes if the front lines spring a leak? Or vise versa ?  

[Gene]

Hi fire797,

 

Confusing is probably a charitable word…

 

Here's my understanding of the situation. With an XJ, the weight distribution, front to back, is essentially constant. It might vary a little, but most of the weight in the back is the weight of the vehicle itself.

 

In an MJ, the distribution of weight, front to rear, can change dramatically. It's a very light vehicle in back without a load, with a full load it becomes much heavier.

 

Ideally, the distribution of braking effort, front to back, should be based on the weight in the front of the vehicle, and the weight in the back of the vehicle.

 

So an XJ can have a fixed proportioning valve, since the weights will not change that much.

 

If an MJ used a fixed valve, it could be set for the unloaded weight, in which case the back would not get the braking that it needs when it's loaded. Or it could be set for a fully loaded rear, in which case the rear brakes would lock up prematurely when not carrying a load.

 

That's the purpose of the height sensing valve in the back, to judge the weight in the back, and to adjust the amount of braking that goes to the back brakes accordingly.

 

But, the problem is, if you lose your front brakes, you want to have absolutely full braking effort going to the rear. That's why the bypass line exists.

 

 

 

My system is stock. I have overload springs on the rear, which give it a pretty good lift, so I think I'm basically just stopping on my front brakes, with very little contribution from the rear.

 

I'm actually thinking of eliminating the rear height sensing valve, and putting an adjustable proportioning valve in its place. I would then adjust this to get good braking with the rear unloaded. I rarely carry heavy loads, so this would not be a big deal for me.

 

If I understand your post, and you're just running a line from front to back, I think you'll be having premature lockup of rear brakes when you're not carrying a load.

 

Hope this helps!

 

Gene

[Joop]

The fsm bleeding procedure is unnessecary tricky. Do a regular bleed on the rear brakes, then loosen the nut on the bypass line under the distribution block and bleed that line, this will push the air the shortest way out and not into the brake cylinders. Then do another one on the rear brakes again just to be shure.

[Eagle]

The fsm bleeding procedure is unnessecary tricky. Do a regular bleed on the rear brakes, then loosen the nut on the bypass line under the distribution block and bleed that line, this will push the air the shortest way out and not into the brake cylinders. Then do another one on the rear brakes again just to be shure.

 

WRONG!

 

Look at the photos. Unless you follow the factory procedure, the shuttle valve will not be tripped and there will be NO FLUID flowing through that fitting. Even if there were, if there's air in the line how is pushing fluid out of the distribution block at the start of the line going to bleed air out of the line?

 

[Edit to add}You'll still have a good pedal if you do what you describe -- because you won't be doing anything to bleed the bypass circuit. Normal brakes will be good. BUT ... if there's air in the bypass line, if the front brakes fail you won't have rear brakes as your safety net, because air compresses and once the bypass circuit opens you'll be compressing the air in that line when you step on the pedal.

[Eagle]

.. But I still have not found an answer to one question I have. Why can't I just replace the distribution block on the MJ with a Propositioning valve from and XJ? Won't this give me the safety of having rear brakes if the front lines spring a leak? Or vise versa ?  

 

You can do that. The XJ proportioning valve also has an internal by-pass (see the photo). The XJ proportioning valve won't be calibrated for an MJ, but it will work.

[dasbulliwagen]

:popcorn:

I saw this thread last night and thought the same thing. Also, this newbie (badguru) has no idea who hes talking to.

[Eagle]

Here's why just disconnecting the nut at the bypass outlet won't accomplish anything. The upper end of the bypass passgae falls between two O-rings on the shuttle plunger when it's in the normal position (centered, so the plunger on the brake warning switch is down). Unless the shuttle is tripped to slide to the right (in the photo), no brake fluid gets to it -- the flow in the normal rear brake circuit just goes past the end of the shuttle plunger and out the "nose" of the distribution block body.

 

The only time brake fluid flows to/through that bypass circuit is when the front brakes fail, causing the shuttle plunger to be pushed to the right (again, as in the picture), which moves the last O-ring beyond the opening to the bypass passage and allows brake fluid to flow through that circuit. (See other photo, above.)

 

[Eagle]

But I still have not found an answer to one question I have. Why can't I just replace the distribution block on the MJ with a Propositioning valve from and XJ? Won't this give me the safety of having rear brakes if the front lines spring a leak? Or vise versa ?  

 

I answered this once, but I don't think I understood the question correctly, so let me try again:

 

Look at the first two photos I posted -- the ones with the valve bodies resting on the cane chair seat. Both the XJ and the MJ units are set up with a bypass circuit that provides full braking power to the rear in the event of the front brake circuit losing pressure. So from that perspective there is no advantage to using an XJ proportioning valve. The advantage to using an XJ proportioning valve is that it allows you to eliminate the rear height-sensing proportioning valve, while still retaining some proportioning. The problem is, as Gene already noted, is that the XJ unit has a pre-set proportioning ratio, and it won't change when the bed of your MJ is loaded down. Of course, it also doesn't change when the back of an XJ is loaded down.

 

On the other hand, I think the XJ curb weight distribution empty has a larger percentage on the rear wheels than the MJ, so the XJ proportioning valve probably allows more braking force to the rear wheels than the MJ height sensing valve does when running empty. So maybe it's not a bad swap.

 

You wrote that you removed the height sensing valve and plugged the outlet to it. Which outlet did you plug -- the one at the "nose" of the distribution block, or the one at the forward bottom (the one labeled emergency bypass in my photos)? The one you need to plug is the one on the bottom. That way, the brake fluid still follows the normal path through the distribution block but, instead of going to the height-sensing valve, it goes directly to the rear wheels -- no proportioning.

 

Which seems to be the answer to your question. If you are concerned about being able to have full power to the rear brakes if the fronts fail, if you eliminate the height sensing valve you will ALWAYS have full braking power to the rears. And that might become a problem if you mostly or always drive with the bed empty, because you might find the rear wheels locking up prematurely. The whole point of a proportioning valve is to prevent premature rear wheel lockup.

 

So what I would say is, try driving it with the MJ distribution block and no height-sensing valve. If you find you're getting a lot of rear wheel lockup, then try an XJ proportioning valve.

[Gene]

Hi everyone,

 

First, I would like to thank Eagle for his very patient discussion above. I think I now understand the MJ rear braking system much better than I had.

 

Second, who has real world experience with this? Anyone eliminate the height sensing valve and just run the brake line directly? Anyone actually running an XJ proportioning valve?

 

If so, any problems with rear wheels locking with an unloaded bed?

 

Thanks

 

Gene

[fire797]

thank you eagle... I pluged the bottom port in the block. Just like the thread in the DIY section said to do. I have added a 4.5 in lift to the truck so I eliminated the height sensing valve. The bed will be unloaded most of the time. And I live in las vegas so I don't think the road will be in a state were the rear wheels will lock up that much..... My next upgrade will be the double diaphragm brake booster up grade. I will try the xj valve at that time.

[Joop]

Eagle, just to clear things up. The whole point by doing my procedure is you don't trip the plunger. Just follow the brakelines on an MJ and imagine how the brake pressure goes and you will see how it will work. In normal operation brake pressure goes out the nose of the block, to the load sensing valve, via the T coupler from the bypass line and out to the wheels. That means the same pressure will also act backwards towards the distribution block through the bypass line, but it has nowhere to go since it end up between the two o-rings you mention. Air trapped here will stay here forever and thats kinda the whole reason behind this discussion. By loosening the nut under the block you will backflush the bypass line, rear to front, using the reduced brake pressure from after the load valve.

[badguru]

Well maybe I don't know whom I'm talking to, but we can deal with that bromance later. Ghetto modifications to your brakes r not good The factory spent a lot of time and money desiging them, so when you kill a school bus load of children being driven by a nun, it goes from accident to murder. Aftermarket brake manufactures have liability on their products. This thread was to have a practical way to bleed ur brakes,and how the rear height sensing valve works. Like comanche afficianado said when that pressure reaches that plunger with the 2 orings it has no place to go well he is right and wrong. Pressure applied to a piston or in this case that plunger that piston or plunger will move (that's how ur engine works) which opens that other line to the braking system increasing overall volume to the rears. In this case the height sensing valve acts as a restricter. pressure remains constant but the hs valve uses volumn to move the plunger until the whole system reaches equilibrium. So depending on where the hs valve is at the time you hit you brake that will determine your overall braking force. That valve works in both direction say your unloaded and slam on the brake nose goes down @$$ end goes up more braking force it is also interactive it will change as you move the hs valve while braking because the orriface size changes as u move the leaver up or down. Remember if you apply 10 psi to a 1 square inch surface it will be half the force of if you apply it to a 2 square surface. Sorry if my writing is bad my keyboard autocorrect is defaulting to max and I cantget it to stay on my settings I'm having to rewrite dam near every word I type

[Eagle]

Hi everyone,

 

First, I would like to thank Eagle for his very patient discussion above. I think I now understand the MJ rear braking system much better than I had.

 

Second, who has real world experience with this? Anyone eliminate the height sensing valve and just run the brake line directly? Anyone actually running an XJ proportioning valve?

 

If so, any problems with rear wheels locking with an unloaded bed?

 

Thanks

 

Gene

 

The rear height-sensing valve in my '88 MJ exploded in a panic stop, so I had no choice -- I had to eliminate it. I plugged the bottom (bypass) outlet on the distribution block and ran a single line from the "nose" outlet directly into the flex hose at the rear axle. I do have a problem with the rear wheels wanting to lock up. Not bad on level ground and dry pavement, but a couple of miles from home there's a 4-way stop intersection that's a steep downhill in the direction I usually travel. I have to be very careful about braking there, because there's even less weight on the rear wheels.

 

Disclaimer: I'm 73 years old. I grew up and started driving decades before proportioning valves were invented, so adapting to rear wheel lockup is an automatic response for me. For you young guys who haven't driven anything that doesn't have brake proportioning, this could be an issue. An option other than using an XJ proportioning valve and hoping it's an improvement would be to use a Wilwood adjustable proportioning valve. That would allow you to dial in the amount of rear braking you're comfortable with, and to increase it if you have to haul a heavy load.