shawn Posted July 6, 2014 Share Posted July 6, 2014 hi guys. wondering if anyone could enlighten me. wifes xj back brakes are not working properly and under fairly hard braking, - like coming to a quick stop down a wet hill on a rainy day it seems like only the fronts are working. ive had this happen on other xj's ive had over the years. i doubt its the distribution box and i believe all the other hardware to be good as well. - any input greatly appreciated. - shawn. Link to comment Share on other sites More sharing options...
Eagle Posted July 6, 2014 Share Posted July 6, 2014 It very well could be the proportioning valve. The XJ proportioning valve is built into the distribution block under the master cylinder. The circuit feeding the rear brakes goes through the front end of the block. Inside, there's a plunger with an O-ring and a very strong spring. Under light braking, there isn't enough pressure to move the spring, so the plunger keeps the rear blocked and the front brakes do all the work. In theory, under heavy braking, there's enough pressure to overcome the spring, moving the plunger and opening the circuit to the rear brakes. My '88 XJ hasn't had any rear brakes for about four years -- I've tested it on snow and ice. NO rear brakes. What I think happens is that, because the rear brakes normally don't get used, the plunger in the proportioning valve gets gummed up, and then when you need it it can't move. I keep meaning to remove the plug from the nose of the proportioning valve and clean it out, but I don't drive the '88 much any more so it hasn't been a priority. Link to comment Share on other sites More sharing options...
cruiser54 Posted July 6, 2014 Share Posted July 6, 2014 Have you tried adjusting the shoes properly? Link to comment Share on other sites More sharing options...
Eagle Posted July 7, 2014 Share Posted July 7, 2014 Have you tried adjusting the shoes properly? That's always a good place to start. It also helps the parking brake to work (a little). Link to comment Share on other sites More sharing options...
shawn Posted July 7, 2014 Author Share Posted July 7, 2014 very plausable. thank you. I'm not familiar with the process but will give an adjustment a try. will read-up on it. prop-valve suggestion also makes perfect sense. it's all good in the dry but i recently found out that on wet streets its a completely different story. :( Link to comment Share on other sites More sharing options...
newbie Posted July 27, 2014 Share Posted July 27, 2014 Hello all, I would like to start by saying thank you for such an informative forum. I have been perusing the forum searching for information on rear brakes. (a subject that seems to be very popular) Just aquired an 86 comanche, i have never owned a jeep vehicle before.. i am having problems with the rear brakes......as in they do not work at all! i removed the rear drums and all is rusted so they have not been working for a while. the height adjuster rod?? is missing from the rear. i have tried bleeding, i got one small squirt of fluid from the r/r and nothing from the l/r. went to the line connections to the wheel cylinders to no avail.. they are frozen and will not turn at all. went to the height thingymajigger and got a dribble from it and a little better pedal but still no rear brakes. is there a way to bypass so i can get the rears working?? thank you for any help you can give..... Jerry Link to comment Share on other sites More sharing options...
Eagle Posted July 27, 2014 Share Posted July 27, 2014 Yes, the bypass for the height sensing valve has been discussed in detail multiple times. It may take a bit of sleuthing, but look around and you'll find multiple threads, some with photos. Link to comment Share on other sites More sharing options...
newbie Posted July 28, 2014 Share Posted July 28, 2014 thanks, will continue the search! Link to comment Share on other sites More sharing options...
Incommando Posted July 28, 2014 Share Posted July 28, 2014 http://comancheclub.com/topic/11127-abandoning-load-sensor-dingy/ http://comancheclub.com/topic/22882-plug-for-the-prop-valve-outlet/#p250570 http://comancheclub.com/topic/4695-mj-to-xj-pop-valve/ It can be as simple as running a new line from the distribution block in front to the soft line connection in the rear and plugging the now extra port in the block. Link to comment Share on other sites More sharing options...
newbie Posted July 28, 2014 Share Posted July 28, 2014 Thank you both for the help, and the links. I have a light work load tomorrow so it will be ipad under the truck time to get an idea of how it is all laid out (depending on the weather) Will try to remember to take pics ,:) Link to comment Share on other sites More sharing options...
Jeep Driver Posted July 28, 2014 Share Posted July 28, 2014 I found no need to run a new line to the rear, I simply flared the existing line. Image Not Found Link to comment Share on other sites More sharing options...
Incommando Posted July 28, 2014 Share Posted July 28, 2014 I keep forgetting that not everyone lives where rust makes replacement a good policy. Link to comment Share on other sites More sharing options...
Eagle Posted July 28, 2014 Share Posted July 28, 2014 I keep forgetting that not everyone lives where rust makes replacement a good policy. Ain't that the truth. Two years ago I bought a used 2001 XJ with the intention that it would be my daughter's when she got her license. The license hasn't happened, and she's away at school, so I've been driving it. The day I got it registered, I slapped the plates on it and drove it to my friend's house (about 3 miles) to show it to him. The brakes felt a bit funky, so we checked the master cylinder. Empty. Well, it was used, and it had 80 something thousand miles, so maybe the previous owner wasn't paying attention. We filled it up, I drove it home (3 miles), and when I got home the MC was empty again. Now ... this XJ was originally sold in PA, spent some time in NY and then some time in NH. It was last titled in Maine before I got it. All rust belt states. I wound up replacing every hard line in the brake system except the short one on the left rear ... all the others were ancient history. Last week I lost the brakes again. I'm sure I don't need to tell you which line let go. The MJ brake lines to the rear like to rust out where they pass between the frame and the gas tank. It's almost impossible to inspect visually. If you don't live in a rust-free area, IMHO it's always wise to just run a new line if you're eliminating the height-sensing valve. It's cheap insurance, because when (not if) it pops you'll be doing it anyway. Link to comment Share on other sites More sharing options...
Incommando Posted July 29, 2014 Share Posted July 29, 2014 When I revamped my MJ I replaced every hard line. I bought a coil of brake line and with the fittings it was about $30 to do this. As you said: cheap insurance Link to comment Share on other sites More sharing options...
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