Pj's tj Murphy nc (proportioning valve o ring removal) - JeepForum.com
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post #1 of 12 Old 06-18-2018, 11:24 AM Thread Starter
33srolling
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Pj's tj Murphy nc (proportioning valve o ring removal)

Tj rear brakes weak to none solution
Easy fix,common problem overlooked.One hard hit on the brake pedal sends the slide in the proportion/combination valve to absolute minimum rear brakes for control purposes.Good theory if you have baby tires that can skid and a lite with min traction.Add big tires and lots more braking is required to stop the Jeep,lots more especially the rear starving for brake pressure.Jack up the rear so the wheels are off the ground,start it up put it in gear,chances are there are no brakes back there.Try bleeding,no to minimum fluid.
The proportion valve below the master cylinder is bad enough stock but a little wear on the oring on the slide totally reduces braking in the rear.Pull the small rubber cover off the front of the proportion valve and put a rag under it,a small amount of brake fluid will come out.Take a 13/16 wrench and remove the spring loaded end,remove the spring and with needle nose pliers pull the slide out.Probly see a messed up oring,remove it anyway and put it back in with the spring.It take strong fingers to get it started.Fill the master cylinder with brake fluid,front reservoir feeds back wheels.Best way to bleed is as follows.Losen the bleeder screws first and tighten slightly.Get 2 ft of aquarium plastic tube like used on fish tanks.Get an empty water bottle and drill a hole just big enough for the tube.Put an inch of brake fluid in the bottle and submerge one end of the tube in the fluid,it must remain submerged.Press the other end of the tube on the bleeder valve and open it.This is how to bleed brakes alone.Start at the passenger rear and pump the brake pedal 3/4 way down about 10 times and check the bottle .Fluid should be showing in the tube.Fill the small water bottle 1/2 way and repeat to left rear.If you did work on the front do them next,passenger front first.Keep an eye on the master reservoir do not let it get close to empty.Have 2 small bottles of dot 3 brake fluid available.Now you have equal braking.There is no control issue during heavy braking cause the big tires do not skid out like the tiny ones.Works great as far as I am concerned especially off road.If you want to decrease braking they sell an adjustable valve add on pretty cheap.This is for a non abs system .Pete

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post #2 of 12 Old 06-19-2018, 07:37 AM
EricD
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Sounds like good info. Your post is a little hard to read though...
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post #3 of 12 Old 06-19-2018, 07:54 AM
biffgnar
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Do you really want equal braking in a TJ? I don't think I do. I agree with improving the fronts to account for bigger tires, but I think the bias is there for real reasons.

Once you open the proportioning valve and remove o ring you have turned it into one circuit? Seems like front bleed shouldn't be an "if".

How would ABS change it?
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post #4 of 12 Old 06-19-2018, 08:05 AM
mrblaine
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Originally Posted by biffgnar View Post
Do you really want equal braking in a TJ? I don't think I do.

Once you open the proportioning valve and remove o ring you have turned it into one circuit? Seems like front bleed shouldn't be an "if".

How would ABS change it?
He didn't open the proportioning valve. He defeated the proportioning section of the combination valve and the front and rear circuits are still intact and discrete. The front portion is only common to the rear section via the shuttle valve and is separated by same unless the seals on the shuttle valve fail.

ABS shouldn't change it since the ABS feed to the rear brakes comes from the combination valve.
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post #5 of 12 Old 06-20-2018, 03:01 PM
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Originally Posted by mrblaine View Post
He didn't open the proportioning valve. He defeated the proportioning section of the combination valve and the front and rear circuits are still intact and discrete. The front portion is only common to the rear section via the shuttle valve and is separated by same unless the seals on the shuttle valve fail.

ABS shouldn't change it since the ABS feed to the rear brakes comes from the combination valve.
Does this increase breaking performance? Is it safe to do assuming everything else in the brake system is working properly? Just curious...
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post #6 of 12 Old 05-12-2019, 08:12 PM
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Originally Posted by cthusker174 View Post
Does this increase breaking performance? Is it safe to do assuming everything else in the brake system is working properly? Just curious...
And here is the 64 dollar question ...Unanswered??
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post #7 of 12 Old 05-13-2019, 06:10 AM
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And here is the 64 dollar question ...Unanswered??
Safe has to be determined by testing. It is a poor decision to try and solve low performance in braking issues from the back of the rig. The fronts do most of the work and if you get the back brakes working much better generally, you will or can start locking them up. That won't be something you would like with your family in the rig on a rain slick highway in traffic. Pay attention to the direction the cup seal sits on the plunger if you want to test it.

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post #8 of 12 Old 05-13-2019, 05:38 PM
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Safe has to be determined by testing.
I hope he/she takes a good video of a panic stop test after removing the differential portion of the combo valve. AND make sure you are alone in the Jeep (the medical bills will be cheaper) on a desolate road and have a tow truck ready when the jeep does circles off the road.... Not only are you sending the fluid to the rear brakes first, now you are sending full pressure to the rears first in a panic stop, the exact opposite of what the combo valve is designed to prevent.

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post #9 of 12 Old 05-13-2019, 05:59 PM
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Originally Posted by jjandascog View Post
I hope he/she takes a good video of a panic stop test after removing the differential portion of the combo valve. AND make sure you are alone in the Jeep (the medical bills will be cheaper) on a desolate road and have a tow truck ready when the jeep does circles off the road.... Not only are you sending the fluid to the rear brakes first, now you are sending full pressure to the rears first in a panic stop, the exact opposite of what the combo valve is designed to prevent.
Thanks for the reply .I am relatively familiar with how a proportioning valve works, That is why I asked that question. There is always advise on these boards of "I heard, I think, Not sure but I think, never owned one but I think" Not interested in opinion or guesses just actual I have done this and no it is not a good Idea because. And the because reasons are exactly as I had figured. So not trying to be an *** I totally appreciate the honest answers.
Thanks Jerry
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post #10 of 12 Old 05-13-2019, 07:00 PM
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So not trying to be an *** I totally appreciate the honest answers.
No problem at all, this kind of "advise" is down right dangerous, not only for the occupants of the Jeep that has been modified, but for those on the road (or sidewalk) around said Jeep.

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post #11 of 12 Old 05-13-2019, 08:08 PM
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Originally Posted by panzerjunky View Post
Thanks for the reply .I am relatively familiar with how a proportioning valve works, That is why I asked that question. There is always advise on these boards of "I heard, I think, Not sure but I think, never owned one but I think" Not interested in opinion or guesses just actual I have done this and no it is not a good Idea because. And the because reasons are exactly as I had figured. So not trying to be an *** I totally appreciate the honest answers.
Thanks Jerry
I can walk you through adding some more pressure to the rear without going full 50/50 on the bias if you promise to tell no one where you got the info.

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post #12 of 12 Old 05-15-2019, 08:21 AM
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Originally Posted by mrblaine View Post
I can walk you through adding some more pressure to the rear without going full 50/50 on the bias if you promise to tell no one where you got the info.
I don't need it but I am curious about your method.

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