Rear brake proportioning thing

[bullyjeep98]

Does anyone have a schematic or drawing how to bypass this thing. It's the valve with the brake lines above the rear axle with a lever on it. It's not hooked to the axle and would like to bypass it but don't know what will happen since it has a return line going back up front.Thanks

[MiNi Beast]

What are you trying to do to bypass it?

[Eagle]

There is a discussion of how to bypass it, complete with photos. Try a search -- you'll need to go back a few months, maybe a bit more than a year.

 

You have been led astray by a common misunderstanding. There is NO "return" line. There is a line that runs from the "nose" of the front metering block to the rear proportioning valve. That line is what you run on under normal conditions, and it provides proportioned braking to the rear wheels.

 

There is another line to the rear, that by-passes the proportioning valve. This is the line that comes out of the bottom of the front metering block. That port is normally blocked, but if the front brakes fail the piston that actuates the brake system warning light also opens the port and allows fluid to flow through the by-pass line, allowing full braking force to the rear wheels.

 

Essentially, what you need to do is remove the line that runs from the bottom of the metering block and plug that outlet. You then want to splice that line into the line coming out of the "nose" of the metering block, so that there is only one line and it goes from the "nose" outlet directly to the hose at the rear axle.

[mvusse]

One question about that:

 

After doing the bypass, if the front brakes fail causing the piston to move, will the brake fluid still go through the front port if the bottom one is plugged? Or will the piston block the front port meaning that when your front brakes fail, you will also not have rear brakes?

[jaekl]

The valve shuttles towards the front line since there is no pressure to balance it and the rear ports still have pressure. Doesn't a XJ metering block reduce the pressure to the rear lines at all times so that the rears don't lock up as easily?

[Eagle]

After doing the bypass, if the front brakes fail causing the piston to move, will the brake fluid still go through the front port if the bottom one is plugged? Or will the piston block the front port meaning that when your front brakes fail, you will also not have rear brakes?

Once you make the modification described, you will ALWAYS have full power braking to the rear wheels. The by-pass port does not block the primary port for the rear wheels. If you find that thread with the photos, there's a shot I took of an MJ front metering block I sliced open to show the way the fluid is routed through the housing.

 

Doesn't a XJ metering block reduce the pressure to the rear lines at all times so that the rears don't lock up as easily?

Yes, and it does it so effectively that most older XJs have no rear brakes at all. I'm considering putting an MJ metering block in my '88 XJ to get some rear brakes. I tested during the winter -- on ICE, if I slammed on the brakes at low speed the front brakes would lock up and the rear wheels just kept turning. I do not consider an XJ proportioning valve a useful swap for an MJ. I would much prefer to deal with having too much brakes at the rear than having none.

[MiNi Beast]

http://www.comancheclub.com/forums/view ... ring+block

 

http://www.comancheclub.com/forums/view ... ck&start=0

 

 

maybe help.

[Eagle]

Also

 

http://comancheclub.com/forums/viewtopi ... ting+valve

[carnuck]

I did the bypass and plugged the other port (capped the second line in case I need it again)

I plan to put in an adjustable valve (from Summit if I can't find one through a NAPA supplier)

 

I just got a '95 ZJ booster (same as XJ booster of similar vintage) that I'm putting in, so I have to match up my system better.

[bullyjeep98]

Thanks guys this is on my trail rig so a little more brake in the rear is good. The lever isn't hooked up and keeps moving around and sometimes causes my rear brakes to drag.

[MiNi Beast]

Thanks guys this is on my trail rig so a little more brake in the rear is good. The lever isn't hooked up and keeps moving around and sometimes causes my rear brakes to drag.

 

 

zip tie :roll:

[bullyjeep98]

I bypassed it today but can't drive it yet got the interior pulled out to bedline it, so hopefully well get to test her out soon.

[Eagle]

I just did the bypass today, too. The rear height-sensing valve was blown out, so I had no rear brakes. I ripped out both lines, except for a new section I had spliced in a couple of years ago in the gas tank area. I plugged the bypass port on the front metering block with a cut-off bolt. Put an O-ring on the bolt, plus a blob of silicone on the nose, so I think the port is pretty effectively closed off.

 

The hard part was snaking a new line down from the metering block. I like to work neat, and to come even close to the original configuration with everything else in place just wasn't going to happen. I finally got it looking semi-presentable, and it doesn't appear to be rubbing against any moving parts, so I think I'm good to go.

 

Got the air box back in, my brother came over to help me bleed the brakes, and just as he's starting to get a peddle -- POP!. There was just ONE section of original line left, not more than a foot long, from the rear hose forward to tie into the section I had previously replaced. Naturally, it blew. So l'il brother went home. I got the old piece out and a new length of line put in, so all should be good to go. If it isn't raining tomorrow afternoon, we'll try bleeding again and hope for the best.

 

It's going to be NICE to have brakes again.

[maddzz1]

There was a good post a while back where someone drew 3 different diagrams. I can't find the post. Does anybody have the link to that post?

 

EDIT: foget it I found it. It is about switching to an XJ prop valve

[Eagle]

I have considered using an XJ proportioning valve. But if that's such a good thing, then why are all the XJ guys taking their proportioning valves apart and removing the spring and O-ring to improve the rear brakes? My '88 XJ has zero rear brakes right now. It has great peddle -- but I tested it on ice during the winter. Coasting down the driveway, I opened the door, leaned out so I could see the wheels, and stomped on the brakes. The fronts locked up and the rear wheels kept right on turning.

 

So I think I'll be happier just doing the bypass on the MJ. I haven't decided yet what to do for the XJ, but I think I'll probably do like everyone else and gut the stock proportioning valve. You don't have to do that with the MJ. Just re-route the lines and "bypass the bypass" and you're done.