sandsgg
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Grimm, Im Going To Go With Your First Post And Say Prop Valve
skidding all over! how does everyone feel abou changing from drums to discs?
- Thread starter xjlady
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CobraMarty
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Just thinking outloud, when that deer jumped out and across and you jammed on the brakes, The master cylinder #1 or prop. valve #2(less doubtful) could have gotten stuck and now only sends fluid to the rear? Do you have ABS? Doubt there is anything mechanically wrong with the rear brakes, they are working fine, all be it, locking up. Doubtful that both rear brake would similtaneously have a mechanical problem. Would need to be a problem further up front that would effect both rears.
xjlady
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- Hopatcong, NJ
according to the papers i got with it i dont believe it has abs. i have the light for it on my dash though. its a 99 sport; did they usualy come with it? its my first jeep.. i dont know much about them
CobraMarty
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I can't post a pic tonight but maybe someone can post a pic of what to look for under the hood to see if you have an ABS module in the back drivers corner under the hood.
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- Rainy side of Washington
Most came without ABS. If you have ABS, you'll see the module with the black motor housing on it between the brake master cylinder and the air filter housing, like this:
And Marty, you are completely wrong about nothing being wrong with the rear brakes causing them to lock up - I've seen a variety of things cause that, all of them things wrong with the rear brakes. In my case it turned out that the self adjusting mechanisms were jam packed with rust and mud and when the drums got wet, they'd slam like that the first 2-3 times I tapped the brakes in the morning. That was on my old 96. Cleaning out the rear drums mostly fixed it.
And Marty, you are completely wrong about nothing being wrong with the rear brakes causing them to lock up - I've seen a variety of things cause that, all of them things wrong with the rear brakes. In my case it turned out that the self adjusting mechanisms were jam packed with rust and mud and when the drums got wet, they'd slam like that the first 2-3 times I tapped the brakes in the morning. That was on my old 96. Cleaning out the rear drums mostly fixed it.
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CobraMarty
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Thanks, just seems low probability that both rear drums would have a mechanical problem at the exact same time.
added- If they lock up, how do they release? Apply the brakes and they lock, release the pedel and the brakes release?
added- If they lock up, how do they release? Apply the brakes and they lock, release the pedel and the brakes release?
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GrimmJeeper
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there are plenty of things that could cause this, the majority of them being a problem inside one or both of the drums. the bottom line is the entire brake system needs looking into. personally I would start furthest from the master cylinder and work my way towards it looking for things like a brake shoe lining that has delaminated, improperly installed shoes, broken springs or faulty adjusters... the list goes on. someone that knows what they are doing needs to look into this.
O-Gauge Steamer
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xjlady, as an FYI, all 97 up XJs have all of the warning indicators in the dash cluster. Only applicable indicators have bulbs. Could be the earlier ones do as well, I just do not have direct experience with them. Can't speak about things about which I do not know...
If you have ABS, the module is hard to miss as kastein has pointed out. Given that ABS works by relieving the brake pressure, it would be difficult to accept an ABS failure as the cause of this issue.
The brakes need to be taken apart by someone who knows what they are doing. IMO, the Heep is unsafe to drive as it sits.
Please keep us in the loop as this is a very rare failure type. It will be nice to know what is the root cause of the malfunction.
If you have ABS, the module is hard to miss as kastein has pointed out. Given that ABS works by relieving the brake pressure, it would be difficult to accept an ABS failure as the cause of this issue.
The brakes need to be taken apart by someone who knows what they are doing. IMO, the Heep is unsafe to drive as it sits.
Please keep us in the loop as this is a very rare failure type. It will be nice to know what is the root cause of the malfunction.
yossarian19
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One of two things are going wrong.
First, your proportioning valve (more accurately, your combination valve) is bad. This valve is in between the master cylinder & the rest of the brake system. It is there to keep the rear drums from locking up and to direct hydraulic pressure to the side that's intact in case of hydraulic failure.
Second, something (could be a lot of things) in the rear brakes. It doesn't need to be both sides that are locking up to cause a skid - I've seen just one corner locked on a lift but when on the ground, it made no difference that the other side was free. This was on a Tacoma - it came in on a tow truck. That bad.
First, your proportioning valve (more accurately, your combination valve) is bad. This valve is in between the master cylinder & the rest of the brake system. It is there to keep the rear drums from locking up and to direct hydraulic pressure to the side that's intact in case of hydraulic failure.
Second, something (could be a lot of things) in the rear brakes. It doesn't need to be both sides that are locking up to cause a skid - I've seen just one corner locked on a lift but when on the ground, it made no difference that the other side was free. This was on a Tacoma - it came in on a tow truck. That bad.
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Actually, the combination valve (you are correct, that's what it should be called, it's 2-3 valves in one) does not connect the rear and front brake circuits at all.
The plunger between the two circuits has O-rings on each end, there's no actual fluid flow through/around it. Its only function is to detect differential pressure (i.e. front brake loses pressure, rear brake therefore forces the plunger over toward the fronts) using the switch to illuminate the brake warning light on the dash.
The combo valve has a front metering valve and a rear proportioning valve to balance the braking force properly. Brake system fail-over is performed by the fact that the master cylinder is a "twin circuit" one, it effectively is two master cylinders in one inline package. The two pistons are balanced using the two return springs inside the cylinder normally, but if either the primary or the secondary loses pressure, the spring collapses and the other cylinder still functions normally. This is why you can still sorta stop if your brakes on one end of the vehicle fail - and why there is a separating wall in the reservoir that ensures you'll have at least a certain amount of fluid still in the working half of the cylinder's reservoir even if the other one fails and drains all its fluid out.
This is also why modifications that involve removing the O-rings from the shuttle plunger that the warning light switch works on are extremely dangerous, they effectively make the brake system back into a single-circuit system. I'm reasonably certain no one actually does this, as I don't think that little plastic warning light switch can actually handle that kind of pressure without immediately blowing out. Removing the O-rings in the proportioning valve at one end isn't a big deal, it basically just disables the proportioning functionality. If the brake system performs adequately with that done, it's perfectly fine.
The plunger between the two circuits has O-rings on each end, there's no actual fluid flow through/around it. Its only function is to detect differential pressure (i.e. front brake loses pressure, rear brake therefore forces the plunger over toward the fronts) using the switch to illuminate the brake warning light on the dash.
The combo valve has a front metering valve and a rear proportioning valve to balance the braking force properly. Brake system fail-over is performed by the fact that the master cylinder is a "twin circuit" one, it effectively is two master cylinders in one inline package. The two pistons are balanced using the two return springs inside the cylinder normally, but if either the primary or the secondary loses pressure, the spring collapses and the other cylinder still functions normally. This is why you can still sorta stop if your brakes on one end of the vehicle fail - and why there is a separating wall in the reservoir that ensures you'll have at least a certain amount of fluid still in the working half of the cylinder's reservoir even if the other one fails and drains all its fluid out.
This is also why modifications that involve removing the O-rings from the shuttle plunger that the warning light switch works on are extremely dangerous, they effectively make the brake system back into a single-circuit system. I'm reasonably certain no one actually does this, as I don't think that little plastic warning light switch can actually handle that kind of pressure without immediately blowing out. Removing the O-rings in the proportioning valve at one end isn't a big deal, it basically just disables the proportioning functionality. If the brake system performs adequately with that done, it's perfectly fine.
xjlady
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- Hopatcong, NJ
I don't have ABS. I didn't have the tome to do it this weekend but next weekend im going to have everything apart. thank you for all of the help!
yossarian19
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- Grass Valley, CA
Advice: only do one side at a time.
The last thing you want is to take both sides apart and then begin to wonder how it all went back together
Each and every last detail is important to proper operation of the brakes.
So take one side apart and leave the other side alone - that way if you need to you can pull the drum off and voila - 3d instruction manual.
The last thing you want is to take both sides apart and then begin to wonder how it all went back together
Each and every last detail is important to proper operation of the brakes.
So take one side apart and leave the other side alone - that way if you need to you can pull the drum off and voila - 3d instruction manual.
Talyn
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Perhaps the previous owner had a leak, fixed the leak, but didn't replace the contaminated shoes.
DanMan2k06
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Ok reviving this thread. Any updates? My jeep does the exact same thing. It's intermittent. Some days the brakes seem normal, other days the rear locks up with little to no pedal force at all. One slight tap and you're leaving black marks.
I've been diagnosing this for weeks now. First started with a disassembly, reassembly, cleaning, and adjusting. No better. Then did the drums (they have a funny warped feeling). No better. Then did the shoes, and exchanged the drums for 2 new ones. No better. Finally, I replaced both wheel cylinders, and the problem hasn't changed at all. The hardware is all recent. I had a few lots lying around and changed the springs etc about a year ago.
I have no ebrake. The cables have been cut, removed, and the brackets that attack to the trailing shoe were also removed. I'm thinking my problem is further upstream somewhere.
I've been diagnosing this for weeks now. First started with a disassembly, reassembly, cleaning, and adjusting. No better. Then did the drums (they have a funny warped feeling). No better. Then did the shoes, and exchanged the drums for 2 new ones. No better. Finally, I replaced both wheel cylinders, and the problem hasn't changed at all. The hardware is all recent. I had a few lots lying around and changed the springs etc about a year ago.
I have no ebrake. The cables have been cut, removed, and the brackets that attack to the trailing shoe were also removed. I'm thinking my problem is further upstream somewhere.
yossarian19
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Dan, I'd think you are looking at a proportioning valve issue. It's supposed to decrease & delay flow to the rear drums so they don't grab like that. What condition is your brake fluid in? Was it ever super cruddy? Poor fluid condition can, over time, cause valve issues.
DanMan2k06
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Nah, fluid has always been changed regularly (probably every 10k, I change it all the time). Vehicle has ABS if it matters, currently disabled because I swapped in an 8.25. How do you go about changing the prop. valve? New master cylinder?
yossarian19
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Hmm. it does matter that yours has ABS. Follow lines from master. If there is a thingie between the MC and ABS, it's your prop valve. If not... Dunno.
old_man
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- Loveland, Colorado
Another problem I have run across is that the PO may have done a brake job and got the leading/trailing brake shoes reversed.
DanMan2k06
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- Damascus, MD
Well guys, hate to say it, but I fixed my problem.... By swapping to discs. I still have absolutely no idea what was causing the issue in the first place, but I think it's poorly made Chinese drum parts to blame. I replaced literally every part inside of them besides the backing plate. Some parts twice. They still jammed and locked up all the time. Usually at random too. I'd brake 10 times fine and the 11th they'd leave black marks.
It's weird getting used to the discs. They actually, work! No adjusting, no maintenance, no parts to get muddy and fouled up and seize. I'm in love.
It's weird getting used to the discs. They actually, work! No adjusting, no maintenance, no parts to get muddy and fouled up and seize. I'm in love.
Vanimal
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bummer you didnt narrow it down. thanks for at least posting up a conclusion though lol
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