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Doin some brake swapping, need guidance


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I will be swapping out the front PV, BB, MC and pedal assembly from a 99 into my 88.

 

Since there are two lines running from the PV to the rear brakes right now, can I just remove the load sensing assy and install a 'T' joint in its place? So the one brake line outlet from the PV will plumb to the 'T', then to the two drums.

 

Does that sound about right?

 

I understand that this will direct the full braking force for the rears (30-40%) at all times, but I'm not too worried about it at this time; I just need to get her up and running (and braking) again.

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Since there are two lines running from the PV to the rear brakes right now, can I just remove the load sensing assy and install a 'T' joint in its place? So the one brake line outlet from the PV will plumb to the 'T', then to the two drums.

 

Does that sound about right?

No.

 

One of those lines is a bypass line. It is normally not used; it comes into service only if/when the front brakes fail. Do NOT interconnect those two lines by removing the height sensing valve.

 

The better approach, if you wish to remove the height sensing valve, is to remove the bypass line entirely. It's the line that comes off the front/bottom outlet on the metering block below the master cylinder. The line that normally provides your braking (through the height sensing valve) is the line that comes out the "nose" of the front metering block. Keep that one, plug the other outlet, run a new, single line from the nose directly to the flex hose at the rear axle, and you're done.

 

BTW -- your terminology is mixed up. In the MJ, the rear, height sensing valve is the proportioning valve. The "thingie" in the front is a distribution block with an emergency bypass circuit. It is NOT a proportioning valve and provides NO proportioning function whatsoever.

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Thanks.

 

I suppose I wasn't clear in the OP. I was planning on removing the emergency bypass line altogether and running only one line to the rear brakes.

 

I'm not very familiar with the set-up in the rear on the MJs and I haven't had a chance to crawl up underneath yet. Does the line from the front distro block run to the height sensing valve, then from there to the wheels? When I run the new line to the rear, will I need to 'T' the line so it runs to the both of the drums?

 

TIA.

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Thanks.

 

I suppose I wasn't clear in the OP. I was planning on removing the emergency bypass line altogether and running only one line to the rear brakes.

 

I'm not very familiar with the set-up in the rear on the MJs and I haven't had a chance to crawl up underneath yet. Does the line from the front distro block run to the height sensing valve, then from there to the wheels? When I run the new line to the rear, will I need to 'T' the line so it runs to the both of the drums?

 

TIA.

The 'T' is on the axle and is the lower end of the flex hose. You run a single line from the front distribution block to the frame in the rear, where the height sensing valve is currently located, and connect that to a new flex hose. (Yes, you can use the existing flex hose -- but it's 20 years old.) The hard line to each wheel cylinder splits off the block where the flex hose bolts onto the axle tube.

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Thanks.

 

I suppose I wasn't clear in the OP. I was planning on removing the emergency bypass line altogether and running only one line to the rear brakes.

 

I'm not very familiar with the set-up in the rear on the MJs and I haven't had a chance to crawl up underneath yet. Does the line from the front distro block run to the height sensing valve, then from there to the wheels? When I run the new line to the rear, will I need to 'T' the line so it runs to the both of the drums?

 

TIA.

The 'T' is on the axle and is the lower end of the flex hose. You run a single line from the front distribution block to the frame in the rear, where the height sensing valve is currently located, and connect that to a new flex hose. (Yes, you can use the existing flex hose -- but it's 20 years old.) The hard line to each wheel cylinder splits off the block where the flex hose bolts onto the axle tube.

 

Gotcha. That makes sense. Is there a reason not to use the existing brake lines that run to the rear, besides them being 20 y/o?

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They're more than likely rusted up along the framerail above the rear axle. It's not a question of if, but rather when, they will pop a leak. Quite a common problem on MJ's, especially if you're in the rust/salt belt area.

 

Jeff

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I know what picture your talking about, I spent 1/2 hour, going back 3 year trying to find it :hmm:

 

It showed a mock up of the MJ with all the brake line drawn in.

 

I found this one -

 

 

It was in the one link I posted.

 

:agree: with the above statement, replace the steel line all the way back. The brake lines tend to rot out behind the gas tank, where you can't see them, don't mater if you in the rust belt or the sun belt, the soft lines are still 20 year old rubber, and if there not dry rotted now, it wouldn't be long before there gone. Your running new lines, you need to bled the system, so why not just replace everything once and enjoy the ride (and the stopping)

 

I need to add something to what Eagle wrote, Yes, there is a "T" on the axle, from the soft line to the axle, to both wheels.

 

But, there is also a "T" fitting up in the frame where the height sensing valve line, and the main line are connected together, leading to the soft line on the axle.

 

When I do my "conversions" I use this T fitting up front at the Distribution valve, and tie the lower front port and the front nose port together, to one line to the rear. I know, you can cut a bolt down and fit it into the lower front port, but I just find this easier to do :hmm:

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I need to add something to what Eagle wrote, Yes, there is a "T" on the axle, from the soft line to the axle, to both wheels.

 

But, there is also a "T" fitting up in the frame where the height sensing valve line, and the main line are connected together, leading to the soft line on the axle.

There is a 'T' in the system now, but if you deactivate the height sensing valve and run a single line from the front (which is what we are discussing in this thread) you don't use that 'T.' The new hard line goes direct from the nose of the front distribution block to the upper (frame) end of the rear flex hose.

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Is there an advantage to using the newer style distribution blocks from an XJ over the older style in the MJ?

 

I would assume that it would work the same, but eliminate the need for custom bent hard lines from MC to the distro block?

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Is there an advantage to using the newer style distribution blocks from an XJ over the older style in the MJ?

 

I would assume that it would work the same, but eliminate the need for custom bent hard lines from MC to the distro block?

Considering that the XJ was introduced in 1983 as a 1984 model and the MJ didn't come out until 1985 as a 1986 model, it's a bit counter-intuitive to refer to the XJ combination valve as "newer." With that:

 

True, the XJ valve has the rear brake outlet on the bottom, which on the MJ is the emergency bypass outlet. There is no "nose" outlet on the XJ block. But if you're replacing both old lines to the rear with one new one, I don't see any advantage to having the outlet on the bottom, where it's harder to see and access, rather than on the front.

 

The XJ combination valve does provide a proportioning function. I have slit one and examined it (same as the photo of the MJ valve above in this thread), and I still can't figure out how it works. Based on the position of the spring and O-rings, it looks like what it does is prevent any braking from getting to the rear except in very hard braking. IMHO that is exactly contrary to what a proportioning valve is supposed to do, which is to prevent premature rear wheel lock-up under heavy braking. Also, the XJ valve is susceptible to gumming up, so that you don't have ANY rear brakes. I have posted elesewhere that in my '88 XJ, if I stomp on the brakes in a snowy parking lot, the front brakes lock up and rear wheels keep turning. Cool -- I'll never spin out, but I'll also never stop.

 

If I were going to install an XJ combination valve in an MJ, I would open it up and gut it. Which, of course, eliminates the primary reason for swapping it in the first place, which would be to get some proportioning. Which, in turn, explains why I have a box with five or six XJ combination valves in it that will probably never be used.

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Is there an advantage to using the newer style distribution blocks from an XJ over the older style in the MJ?

 

I would assume that it would work the same, but eliminate the need for custom bent hard lines from MC to the distro block?

Considering that the XJ was introduced in 1983 as a 1984 model and the MJ didn't come out until 1985 as a 1986 model, it's a bit counter-intuitive to refer to the XJ combination valve as "newer." With that:

 

True, the XJ valve has the rear brake outlet on the bottom, which on the MJ is the emergency bypass outlet. There is no "nose" outlet on the XJ block. But if you're replacing both old lines to the rear with one new one, I don't see any advantage to having the outlet on the bottom, where it's harder to see and access, rather than on the front.

 

The XJ combination valve does provide a proportioning function. I have slit one and examined it (same as the photo of the MJ valve above in this thread), and I still can't figure out how it works. Based on the position of the spring and O-rings, it looks like what it does is prevent any braking from getting to the rear except in very hard braking. IMHO that is exactly contrary to what a proportioning valve is supposed to do, which is to prevent premature rear wheel lock-up under heavy braking. Also, the XJ valve is susceptible to gumming up, so that you don't have ANY rear brakes. I have posted elesewhere that in my '88 XJ, if I stomp on the brakes in a snowy parking lot, the front brakes lock up and rear wheels keep turning. Cool -- I'll never spin out, but I'll also never stop.

 

If I were going to install an XJ combination valve in an MJ, I would open it up and gut it. Which, of course, eliminates the primary reason for swapping it in the first place, which would be to get some proportioning. Which, in turn, explains why I have a box with five or six XJ combination valves in it that will probably never be used.

 

I assumed that by 1999, Jeep would have changed the design of the front PV through the years to tweak it so it works perfect. Apparently, they didn't do that.

 

Looks like I won't need to swap that over in the end.

 

Thanks for everyone's help. I have a buddy coming over today to help me tackle this project. I'll post back if something catches on fire.

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