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MJ Brakes OMG! :-)


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So I know,  There is a lot of topics and conversations around the brakes and I think I now have read them all but now I think I'm more confused then before I started researching.  We are rebuilding a 88 4.0L 4wd.   New brake lines hard and flex, new calipers and new master cylinder.  My problem is no pedal & I can't get the front calipers to bleed.  The back no problem they bleed fine with or with out the LSV which now after a multitude of tries I have temporarily routed around trying to get the fronts to bleed.   What seems to be happening is when we go to bleed the fronts by Pedal or by Vacuum the proportioning valve moves to the rear and shuts off the Right front and limits the left front.   Has anyone seen this before? Based on the blogs that seems to be what its designed for?  Is there a simple fix so I can bleed the front?  Should I keep the MJ PV or should I just get a YJ/XJ PV and be done with it and if so does anyone have a Part number of one YJ/XJ that will hook up easily in that small hand space.LOL     I would like to Keep the LSV but honestly this truck is not going to be carrying loads it will be lucky to have a full tank of gas. it will be for weekend warrior stuff.  So eliminating the LSV  I don't think it would be an issue if there is a PV thats close to the standard proportions of the settings on the proportioning valve with LSV.  

 

Owe and yes we tried the MJ brake bleeding procedure with no luck.  tried starting at rear, tried doing front first, tried the one were we leave the right front open and bleed the back.  no luck

 

Thanks in advance....

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The proportioning valve also contains two features mandated by Federal motor vehicle safety laws:

 

1.  a design that separates the front and rear brake systems so that one system can be open (leaking) and leave the other system capable of providing braking function.  This is accomplished by having a separate piston and output for the front and rear brake systems in the MC, and they are routed to the prop. valve and theron to the wheels.

 

2.  an electrical switch on the prop. valve that will close to turn on a light on the dash when one of the two systems develops a leak that causes an imbalance in pressure from front to back.

 

The prop. valve also will restrict/reduce pressure to the rear brakes to prevent premature locking of the brakes due to excessive line pressure to the back.  It does this with a spring and and O-ring inside the valve, and is normally NOT adjustable.

 

The fact that you are opening the fronts and getting them to virtually shut off makes me think your prop. valve is defective.  Either it's gunked up, or missing it's O-ring, or missing it's spring.

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1 hour ago, AZJeff said:

The proportioning valve also contains two features mandated by Federal motor vehicle safety laws:

 

1.  a design that separates the front and rear brake systems so that one system can be open (leaking) and leave the other system capable of providing braking function.  This is accomplished by having a separate piston and output for the front and rear brake systems in the MC, and they are routed to the prop. valve and theron to the wheels.

 

2.  an electrical switch on the prop. valve that will close to turn on a light on the dash when one of the two systems develops a leak that causes an imbalance in pressure from front to back.

 

The prop. valve also will restrict/reduce pressure to the rear brakes to prevent premature locking of the brakes due to excessive line pressure to the back.  It does this with a spring and and O-ring inside the valve, and is normally NOT adjustable.

 

The fact that you are opening the fronts and getting them to virtually shut off makes me think your prop. valve is defective.  Either it's gunked up, or missing it's O-ring, or missing it's spring.

 

That front block in the MJ is NOT a proportioning valve, and it does not contain a plunger or spring. The XJ unit is a proportioning valve and meets this description, but not the MJ. All proportioning in the MJ is done by the rear load/height-sensing valve.

 

The shuttle valve in the front metering block should not move if you vacuum bleed one wheel at a time. It also should not move bleeding by foot pressure unless you're pushing too hard and too fast.

 

This is the MJ metering block, cut open (this shows it with the shuttle valve tripped, to open the rear brake bypass circuit):

 

image.jpeg

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13 hours ago, Eagle said:

 

That front block in the MJ is NOT a proportioning valve, and it does not contain a plunger or spring. The XJ unit is a proportioning valve and meets this description, but not the MJ. All proportioning in the MJ is done by the rear load/height-sensing valve.

 

 

My error in passing out this information.  I now see that my MJ has an XJ prop. valve in it (from a previous owner), and I assumed all MJ's had that valve as well.

 

SOrry to anyone I led astray.   Eagle's information is to be followed, not mine.

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Have you tried leaving a front bleeder open while you bleed the rear? 

From factory there's two lines going to the back, one is the line going through the load valve, the other is the emergency bypass being talked about. Factory bleed procedure is to initially bleed the rears with a front bleeder open to simulate a failure so you can bleed the bypass, then close it and bleed rears again, then the fronts. 

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Eagle,    

 

So I have tried a lot of different ways to to get the fronts to bleed but here are a few. So I originally didn't replace the front Calipers or Master cause when we got the MJ it had brakes just not great and the brake lines were a mess.  So we replaced the two lines going to the rear LSV and along the rear axel to the wheel cylinders. We did a 5 "lift and replaced the front Flex lines to the Calipers we used Rough Country braided lines.  With the LSV lined in my only question there was did I get the lines going into the proper ports? My diagram is above in the thread.  

So the first Bleed attempt was a standard try Pump the brea pedal normally 4 to 5 times hold to the floor crack the Rear Right Bleeder 1/4 turn I used a Clear line into a Bottle to check for air. Close the leader and repeat til we had no air visible in the line while checking the master calendar fluid level so it did not go low enough to add additional air.  Went to the rear left and performed the same steps.  We started with absolutely no pedal and after the rear line bleeding we had a small pressure on the pedal but if you put a solid push on it you could push it to the floor.  We then moved to the right front and started the bleeding process same as above pump 4 to 5 times hold down on pedal release check for air then tighten and repeat. so after a few releases of air I noticed the amount of fluid had decreased and no air bubbles were moving so we moved to the left front and performed the same.  We got a ton of air out but it wouldn't stop and we had lost all pedal that we had gained from the rear bleeding. we were also having at times when bumping up then holding the pedal hold off the floor a bit then for no reason the pedal release and go to the floor before we touched a bleeder.  

We tried this method a couple times then followed the process that gogmorgo mentions above by leaving the RF bleeder open with no luck.    at this point I bought and install New not rebuilt Master and calipers.  I bench Bleed the Master and installed the calipers bleeder at top.  Started at the front just to see if we could get it all primed and some type of pedal, we had no luck. I still had no pressure at the RF caliper. I removed the Flex line and connected a clear runner hose to the hard brake line in the wheel well. Connected a Large syringe to it and tried to pull fluid through the system. This is when I realised the line was totally blocked off. We removed the line from the rear of the metering valve and checked line, line was clear.  We removed the metering valve rear port on metering valves was blocked hundred percent rear bottom port had major restriction the front two ports were wide open. Disassembled metering valve extremely clean inside seals all looking good shape repositioned valve in valve body check for airflow and reinstalled.  

 

Started the bleeding process this time with the front right caliber open and right rear leading process again got partial pedal went to bleed to front and the metering valve shut off again.

So thinking the LSV is shutting off the front lines I remove the LSV from the system and routed lines around it and tried bleeding process again with same outcome soon as we tried to bleed the front it locked up.  I gave up after a multitude of tries bleeding rear first, front first, etc. 

Open to any suggestions.  

 

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When bleeding by the pedal method, is the "pedalman" (for lack of a better term) pressing slowly, or is he pushing really hard? The shuttle shouldn't trip from bleeding brakes. I've never heard of that happening, so my first thought is that the pedalman is just pushing too hard. But -- one thing you might try is to reset the shuttle, leave the brake light switch out of the metering block, and have a second helper manually block the shuttle from moving by holding a piece of metal rod (perhaps the back end of a drill bit that matches the plunger hole diameter) against the "waist" of the shuttle while you and another assistant do the bleeding. Or stick a plunger substitute in there and hold it with a hose clamp.

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