Brake bleeding

[Lthompson741]

I know this has been beat to death but at my wits end.   

88 4wd 4.0

I’ve got it with little to no brakes.   The P.O. Replaced the rear line and the return.    The “brake”light is on.    It will stop, eventually, it feels like it needs to be bled. I am not getting air anymore, just straight clean fluid.   It appears on only be drawing fluid for the rear out of the front smaller compartment of the master cylinder.  I should mention I replaced the rubber line in the rear, and the hard lines on the axle due to there questionable rust/cracking.   

 

I  bled almost a gallon thru all four wheels, in the proper order and can not get a decent pedal.   My shoes are adjusted on the rear, I hear them dragging.

 

I sometimes can get decent pressure to the rear, other times it just dribbled out.  If you pump the pedal up you can get them to work but it is very inconsistent.       Running it pretty much goes to the floor, but you can pump them up and get them to engage.  

Does this sound like my master cylinder is going out?  Or am I missing something.   I have looked into the proportioning valve being stuck, in emergency mode, but I can get enough pressure to recenter.  It I unplug the wiring harness my “brake” light goes out.    I'm sick of replaceming parts that aren’t needed and it’s another $50 bucks on a mj that cost me $500 and I’m starting to realize why it was parked.

 

sorry for the long rambling post, just discouraged.    

[gogmorgo]

Do you still have the load sensing valve in the picture? 

You can just barely see it, the rod coming up off the left hand side of the diff, and the bar going down and left from the top of that. That's the way it's supposed to be set up, but sometimes they will over-centre themselves, and the bar ends up pointing downwards, which significantly reduces pressure to the rear brakes. 

Proper bleeding procedure is to crack open a front bleed screw and then bleed rear right then left with the front screw open, then close the front screw and bleed rear right and left again, then front right and left. 

 The shuttle valve that triggers the brake warning light moves to provide full braking pressure around the load sensing valve if there's uneven pressure between the two, essentially a front brake failure. Opening the front bleeder simulates a front brake curcuit failure, shifting pressure to the rear, allowing you to bleed out the backup line. If that line has air in it, the shuttle valve will always move over and you won't have much for brakes. Once you have that line bled out, the system will function normally, allowing you to bleed normally. 

[Eagle]

10 hours ago, Lthompson741 said:

 

I’ve got it with little to no brakes.   The P.O. Replaced the rear line and the return.    The “brake”light is on.    It will stop, eventually, it feels like it needs to be bled. I am not getting air anymore, just straight clean fluid.   It appears on only be drawing fluid for the rear out of the front smaller compartment of the master cylinder.  I should mention I replaced the rubber line in the rear, and the hard lines on the axle due to there questionable rust/cracking.   

 

 

There is no "return" for the rear brake circuit. There are many internet sources that claim this, but it's incorrect. The second line to the rear is an emergency bypass line. The purpose is to bypass the load-sensing proportioning valve  and allow full brake pressure to the rear if the front brake circuit fails. Under normal conditions, no brake fluid ever flows through that bypass circuit, but it's there and it must be bled if you want the rear brakes to function properly. There is a special procedure in the FSM for bleeding the brakes in an MJ with the load-sensing proportioning valve.

 

https://comancheclub.com/topic/4647-brake-bleeding/

 

https://comancheclub.com/topic/40428-brake-bleeding/?tab=comments#comment-407703

 

Also, the smaller (front) reservoir in the master cylinder is the one that supplies the rear brakes, so the fact it appears to be drawing fluid from there is entirely correct.

[Lthompson741]

10 hours ago, gogmorgo said:

Do you still have the load sensing valve in the picture? 

You can just barely see it, the rod coming up off the left hand side of the diff, and the bar going down and left from the top of that. That's the way it's supposed to be set up, but sometimes they will over-centre themselves, and the bar ends up pointing downwards, which significantly reduces pressure to the rear brakes. 

Proper bleeding procedure is to crack open a front bleed screw and then bleed rear right then left with the front screw open, then close the front screw and bleed rear right and left again, then front right and left. 

 The shuttle valve that triggers the brake warning light moves to provide full braking pressure around the load sensing valve if there's uneven pressure between the two, essentially a front brake failure. Opening the front bleeder simulates a front brake curcuit failure, shifting pressure to the rear, allowing you to bleed out the backup line. If that line has air in it, the shuttle valve will always move over and you won't have much for brakes. Once you have that line bled out, the system will function normally, allowing you to bleed normally. 

Yes, the load sensing valve is in place.   The rod was broken when I got this thing, so currently it is wire tied up.   

 

I tried leaving the the front cracked several times, and get decent pressure there but it makes not difference that I hav seen yet to the back pressure.

[gogmorgo]

Well the master cylinder could be on its way out then. You could try bleeding it specifically before swapping out. Either bench bleed it with outlet lines directed back into the open reservoirs, or just do it by cracking the lines on it, same principle as bleeding at the bleed screws, get a helper to pump the brakes as you crack and tighten the lines. Wear eye protection and use a rag to catch as much of the fluid as possible, and then hose down the engine bay really well with water if you like having paint on things in there. 

[Lthompson741]

2 hours ago, gogmorgo said:

Well the master cylinder could be on its way out then. You could try bleeding it specifically before swapping out. Either bench bleed it with outlet lines directed back into the open reservoirs, or just do it by cracking the lines on it, same principle as bleeding at the bleed screws, get a helper to pump the brakes as you crack and tighten the lines. Wear eye protection and use a rag to catch as much of the fluid as possible, and then hose down the engine bay really well with water if you like having paint on things in there. 

I’m thinking it’s the master cylinder.    I can’t get enough pressure to get a proper bleed.   I went to harbor freight today and got a suction bleeder and keep sucking quarts thru each wheel in order.      It seems a little better driving it around but no where near safe.    I just stripped off the blender in one of my wheel cylinders so I give up for the day.    So I need to either find a new bleeder or a new wheel cylinder.     I’m thinking I’m gonna order a master cylinder as well.     

[Eagle]

7 hours ago, Lthompson741 said:

Yes, the load sensing valve is in place.   The rod was broken when I got this thing, so currently it is wire tied up.   

 

I tried leaving the the front cracked several times, and get decent pressure there but it makes not difference that I hav seen yet to the back pressure.

 

But did you verify that the brake warning light was on, which is the way you verify that the emergency bypass circuit has opened? Both links I provided above spell out a very specific procedure for properly bleeding the MJ brakes. If your procedure didn't follow those steps -- in the same sequence -- your brakes aren't properly bled regardless of how much fluid you pushed (or pulled) through the system.

[Lthompson741]

2 hours ago, Eagle said:

 

But did you verify that the brake warning light was on, which is the way you verify that the emergency bypass circuit has opened? Both links I provided above spell out a very specific procedure for properly bleeding the MJ brakes. If your procedure didn't follow those steps -- in the same sequence -- your brakes aren't properly bled regardless of how much fluid you pushed (or pulled) through the system.

I guess I’ll try again, I never caught the part about moving the height sensing valve.   My “brake” light is on, and has been on sense I first got it running.    

 

-Ive bleed all wheels In order

-Bleed both rear wheels with a front bleeder cracked as well.

-i have done it via the pumping method, and also with a suction bleeder.

 

i can not get enough steady pressure via the pedal method to the back wheels.  

 

My my question is how do you reset the system,  or bleed it when it is already in the brake failure mode.

 

is it possible for the proportioning valve to be stuck in the emergency position?

 

ive read every thread about bleeding this system and am nowhere closer to solving this.    

 

I will rebleed it this weekend hope for the best.

 

 

[Eagle]

The light being on (if the parking brake is off) indicates that the shuttle valve has tripped to the bypass position. Bleed the rear brakes with a front bleeder left open when it's like that.

 

Once you have done that, close the front bleeder that was open, and then stomp sharply on the brake peddle a couple/few times. That should reset the shutle valve to the normal position. If it doesn't, re-bleed the front caliper you left open, and repeat the stomping. You MUST reset the shuttle valve before you can complete bleeding the rear circuit.

 

Here's a photo of what the front distribution block looks like inside:

 

 

In this photo the shuttle valve is in the normal position -- centered. The plunger for the switch is down, in the "wasp waist" portion of the shuttle plunger. If the front brakes fail, the plunger moves to the rear (to the right, in the photo). The black O-ring farthest to the left moves past the point labeled "Emergency Bypass Circuit," opening that vertical passage. Once that circuit nas been bled, you need to close that passage again to properly bleed the normal rear brake circuit.

[Eagle]

One further thought -- if the previous owner replaced both lines to the rear, you should confirm that he didn't get them crossed. The one from the "nose" of the front distribution block is the normal circuit. That line should go to the height/load sensing valve mounted to the frame above the differential. The line from the bottom/front outlet of the distribution block goes to the tee fitting adjacent to the rear height sensing valve.

[Lthompson741]

9 hours ago, Eagle said:

One further thought -- if the previous owner replaced both lines to the rear, you should confirm that he didn't get them crossed. The one from the "nose" of the front distribution block is the normal circuit. That line should go to the height/load sensing valve mounted to the frame above the differential. The line from the bottom/front outlet of the distribution block goes to the tee fitting adjacent to the rear height sensing valve.

I actually had the same thought last night.  I’ll check after work.

10 hours ago, Eagle said:

The light being on (if the parking brake is off) indicates that the shuttle valve has tripped to the bypass position. Bleed the rear brakes with a front bleeder left open when it's like that.

 

Once you have done that, close the front bleeder that was open, and then stomp sharply on the brake peddle a couple/few times. That should reset the shutle valve to the normal position. If it doesn't, re-bleed the front caliper you left open, and repeat the stomping. You MUST reset the shuttle valve before you can complete bleeding the rear circuit.

 

Here's a photo of what the front distribution block looks like inside:

 

 

In this photo the shuttle valve is in the normal position -- centered. The plunger for the switch is down, in the "wasp waist" portion of the shuttle plunger. If the front brakes fail, the plunger moves to the rear (to the right, in the photo). The black O-ring farthest to the left moves past the point labeled "Emergency Bypass Circuit," opening that vertical passage. Once that circuit nas been bled, you need to close that passage again to properly bleed the normal rear brake circuit.

Yeah the light is on due to the emergency circuit, I verified it a couple days ago.     I’ll hit it all again tonight and see if I can make some progress.

[Lthompson741]

Well, did the bleed process to the “t” and no joy. I can not get it to switch back out of emergency mode.      

 

I also put put a new bench bled master cylinder on it.    It seems as tho the pressure has increased but no where near where it should be.  

 

Ive reviled the front caliper over and over again.   I’ve stomped so hard on the brakes my legs are tired.    

 

So my next choice is what?   Replace the valve?   Is it worth it to replace it with a Cherokee one?      If so what year is a direct replacement?

 

thabks for all of your help.

Edited February 23, 2019 by Lthompson741
Adding info

[zomeizter]

If you're certain it's in bypass mode and the pedal stomping is not resetting the valve, maybe it's time to remove the valve from the distribution block and check for damaged o-rings, maybe it's stuck with contamination. When I removed this valve in my 'manche, I could not believe the amount of crud on it, I guess it was due to it sitting unused for 10+ years...

[Eagle]

41 minutes ago, zomeizter said:

If you're certain it's in bypass mode and the pedal stomping is not resetting the valve, maybe it's time to remove the valve from the distribution block and check for damaged o-rings, maybe it's stuck with contamination. When I removed this valve in my 'manche, I could not believe the amount of crud on it, I guess it was due to it sitting unused for 10+ years...

 

That's a very good possibility. The distribution block isn't difficult to work on -- the hardest part will be getting it out of the vehicle. Give it a good cleaning out and see if that makes the difference.

[Lthompson741]

9 hours ago, Eagle said:

 

That's a very good possibility. The distribution block isn't difficult to work on -- the hardest part will be getting it out of the vehicle. Give it a good cleaning out and see if that makes the difference.

Well I tore it out today.     It was a rusted blob inside.   It is rusted solid.    I ended up going to the junk yard and grabbed one out of a 1989 xj.   And installed that one removing the load lever/ emergency circuit.

 

it was a pain and I stripped every connection at the block.  (I now have all new brake lines.). 

 

Best news yet is that it stops.    Like crazy good, it will throw you into the windshield.

 

it does lock the backs up, which I kind of figured. Only if I stand on it tho.     But it is also pouring rain here.

 

i appreciate all the help.!!

[zomeizter]

Right on man, glad you got it resolved...

[Lthompson741]

Yeah me too blew my budget gettin it on the road but it’s drivable now.   Now onto kruiser’s electrical refreshing

 

Bonus picture of the tucked valve.