chopper35nj Posted November 13, 2012 Share Posted November 13, 2012 I have been having problems with my brakes since I got this truck and just plain got tired of messing with used parts, so .... I replaced ALL brake lines, calipers, cylinders, master cylinder and booster. I figured that way I would have a new brake system good for another 100K and just be done with it. The only piece that I haven't replaced with new is the metering valve, I did however swap in a metering valve from a 96 XJ with about half the miles as my Comanchee, I did this because the shuttle valve in the stock metering block would not move freely. The load sensing valve was removed from the truck when I got it and the bypass line outlet hole in the metering block plugged with a brass plug. I have been trying to get this system blead for quite a while, so I bought a vaccuum bleader and hooked it up and it works realy well, when finnished I had great pedal after pumping three times but when I start the engine the pedal go's to the floor, when I shut it off the pedal does not rise, and the booster holds vaccuum telling me that the booster is working. When I drive the truck I am not totaly without brakes though but what I do get is way down at the bottom of the pedal stroke and tends to lock up the rears and the turns on the "BRAKE" light on the dash. I am at the point that I want to start driving this truck on the street but not like this, safety is a great concern to me. Just so you know, yes I did bench bleed the master and it was rock hard and I can't find any leaks in the system. Any help would be greatly appreciated. Thanks Eric Link to comment Share on other sites More sharing options...
Eagle Posted November 13, 2012 Share Posted November 13, 2012 On the front of the XJ proportioning valve there's a plug, with a vent hole through it and a rubber cap over the vent. Under that is a spring-loaded plunger that does the actual proportioning. The plunger has a rubber O-ring on it. That whole plunger/O-ring gizmo sometimes gets gunked up and won't move, which results in no brakes at the rear. I would start by CAREFULLY (that spring is strong) removing that assembly and swabbing out the bore with brake clearer and Q-Tips. Clean up the plunger, spring and O-ring, then reassemble, re-bleed, and try it. Link to comment Share on other sites More sharing options...
chopper35nj Posted November 13, 2012 Author Share Posted November 13, 2012 Ok, that sounds like a good idea and I will do that. I do however have rear brakes a little too much actually, they lock up and that could be the proportioning valve malfunctioning. My big problem is maintaining pedal pressure though. Link to comment Share on other sites More sharing options...
NotMatt Posted November 14, 2012 Share Posted November 14, 2012 I know you said you replaced all the wheel cylinders, but I would double-check the rears after driving it for a bit and see if they are leaking. What I've seen happen is that the wheel cylinders start leaking a little bit onto the inside of the drum, and the brake fluid will actually cause the drum brakes to lock up easily as if they were being over-pressurized. You'll notice this about a rear wheel cylinder leak FIRST before you see any brake fluid leaking out from the drum or coating the inside of your tire. After enough fluid leaks out, the shoes get coated and they stop doing this, but eventually so much fluid has leaked out that the brake pressure switch gets tripped (turning the brake light on) every time you step on the pedal. I would also do like Eagle said and look really carefully at the internals in that prop block, and then investigate for leaks elsewhere in the system. The brake light coming on is an indication of a pressure imbalance front to rear in the system, whether it's right near or inside the prop block, or all the way back at the wheel cylinders is anyone's guess without looking at the whole system. You could also have a leak in the front, which would cause the rears to want to lock up easier since there's no stopping assistance from the front brakes. Link to comment Share on other sites More sharing options...
chopper35nj Posted November 14, 2012 Author Share Posted November 14, 2012 Again, good advice. Thanks I'll check it all out.................. AGAIN!!!!! Eric Link to comment Share on other sites More sharing options...
chopper35nj Posted November 14, 2012 Author Share Posted November 14, 2012 OK, I did all that. I disassembeled and cleaned the prop block, it was surprisingly clean but it is now spotless, all O-rings are in good shape and the shuttle valve moves freely. I removed the rear drums and looked for leaks at the cylinders, nuthin'. Falling into the "while I'm here" category I cleaned the rear brake system and pulled the adjusters apart, wire brushed and anti siezed them, reassembeled the brakes and adjusted them. I then cleaned the entire brake system with brake parts cleaner, and I do mean "the entire system" end to end all the lines, fittings, etc. I vaccuume blead the system at all four corners, got in and pumped the peddal twice and had rock hard peddal, had a look around, can't find a single leak..... lookin' like I have this beat, right?..... Nope, start the engine and the peddal go's streight to the floor. What the hell? I am so frustrated. I have never experienced anything like this. Any other ideas????? Eric Link to comment Share on other sites More sharing options...
Eagle Posted November 14, 2012 Share Posted November 14, 2012 Ok, that sounds like a good idea and I will do that. I do however have rear brakes a little too much actually, they lock up and that could be the proportioning valve malfunctioning. My big problem is maintaining pedal pressure though. I still think it's related to the XJ proportioning valve. One of my NAXJA friends always had a very soft peddle in his XJ trail rig. He replaced just about every possible component in the system (except that proportioning valve) to no avail. Finally, he ripped that out and installed a Wilwood manually-adjustable proportioning valve in the line to the rear, and that cured his problem. I don't like the XJ proprtioning valve in XJs, and I think it's a huge mistake to put one on an MJ, but you've already done it so we have to go forward from there. Link to comment Share on other sites More sharing options...
Pete M Posted November 14, 2012 Share Posted November 14, 2012 my 88 had a stiff pedal with the engine off, but then the pedal would plunge to the floor if I turned the engine on. it turned out to be a bad booster. I know you've replaced it, but it's always possible that replacement parts go bad. :( Link to comment Share on other sites More sharing options...
chopper35nj Posted November 14, 2012 Author Share Posted November 14, 2012 In all of this I am using my old XJ as a bench mark, my daughter has it and drives it every day. The pedal in that Cherokee is perfect so I had no reason to think that this would be any different. Wouldn't the need for a prop valve to the rear indicate that the rear brakes are maladjusted? I think re-replacing it might be the next step, good thing I keep reciepts. Thanks Eric Link to comment Share on other sites More sharing options...
Comanche County Posted November 14, 2012 Share Posted November 14, 2012 Did you bleed the master cylinder? It can be done in the vehicle just remove the hard lines, get two extra hard lines, attach them, then route them back up into the MC reservoir. Pump away till no more bubbles. If you didn't bleed the MC, I'd bet that's where your problem lies. A new MC must be bled before the lines to each wheel. Link to comment Share on other sites More sharing options...
Eagle Posted November 15, 2012 Share Posted November 15, 2012 Wouldn't the need for a prop valve to the rear indicate that the rear brakes are maladjusted? No. Proportioning valves are used to alleviate having too much braking at the rear, with resultant premature rear wheel lockup and spin-outs. The easy solution is to use very wimpy (or no) brakes on the rear -- but then you have insufficient brakes when the vehicle is fully loaded. Link to comment Share on other sites More sharing options...
maddzz1 Posted November 15, 2012 Share Posted November 15, 2012 If you scroll down to the last post on this page it shows the correct line routing when eliminating the load sensing valve and keeping the MJ proportioning valve. Not sure if the routing from the proportioning valve would be different if you are using the XJ prop valve. If you have the lines routed this way and have bled the MC and its still not working, I would swap the MJ prop valve back in and then see what happens. http://comancheclub....picture-thread/ Link to comment Share on other sites More sharing options...
Eagle Posted November 16, 2012 Share Posted November 16, 2012 If you scroll down to the last post on this page it shows the correct line routing when eliminating the load sensing valve and keeping the MJ proportioning valve. Not sure if the routing from the proportioning valve would be different if you are using the XJ prop valve. If you have the lines routed this way and have bled the MC and its still not working, I would swap the MJ prop valve back in and then see what happens. http://comancheclub....picture-thread/ The routing has to be different with the XJ proportioning valve. With the MJ metering block, if you eliminate the rear proportioning valve the new single line to the rear comes out the nose of the metering block. The XJ proportioning valve doesn't have an outlet there. For the XJ proportioning valve, the rear brakes get connected to the port you would plug on the MJ metering block. (Bottom/front) Link to comment Share on other sites More sharing options...
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