rear discs so so

[pokeyxj]

I converted my 01 xj to rear discs using the rear set up from a 96 ZJ including swapping the prop valve internals. Everything went pretty smooth. I ran the XJ while the rear was up on jackk stands, the rear brakes worked. The real test is driving. The rears will not lock up like the fronts. I bled the rears again with no improvement. Any ideas. Did I over look something?

 

[lazyxj]

The rears will not lock up like the fronts. I bled the rears again with no improvement. Any ideas. Did I over look something?

Drive it for a while to let the new pads bed themselves and re-bleed it in a day or two. The proportioning valve is suppose to help eliminate rear wheel lockup. Try removing the piston to see if that helps.
FYI, it's preferable to have the front wheels lockup before the rears do for better directional control. In wet weather, early rear wheel lockup can cause a spin.
ZJ rear discs are not the major upgrade many think they are going to be, why? Because most braking is done by the front wheels. Their major advantage is no adjustments and better performance in wet weather.
If you get larger front brakes, rear wheel lockup will become more common, since more weight will be transferred to the front end under hard braking, unloading the rear wheels.
My XJ's rear discs lockup easily ever since the 48mm Vanco front brakes were installed. An adjustable proportioning valve may be the fix to eliminate spooky braking in the wet.

 

[Outlander560]

i might be lost hear but why do you want your wheels to lock up

 

[pokeyxj]

Since I do not have ABS on the 01 the wheel will skid when braking hard. I thought the disc conversion should do the same but would have the advantages lazyxj stated. I thought about using the xj prop valve internals to see the difference.

 

[Talyn]

The ability to lock or not lock your wheels tests and proves nothing.

 

[jbmoose]

the rear discs conversion greatest improvements were stated previously. If you want rear lock up, keep the rear drums and do a few water crossings.

 

[XJEEPER]

I converted my 01 xj to rear discs using the rear set up from a 96 ZJ including swapping the prop valve internals. Everything went pretty smooth. I ran the XJ while the rear was up on jackk stands, the rear brakes worked. The real test is driving. The rears will not lock up like the fronts. I bled the rears again with no improvement. Any ideas. Did I over look something?

What was your bleeding procedure?

I have TJ rear discs on my D44 equipped non-ABS 2000, with WJ discs up front and TJ prop valve guts. I prefer my rig to be controllable in a panic stop situation, instead of having the tires locked up and skidding....this exact scenerio led to the invention of ABS. Did you perform this procedure after installing your brakes? Works, regardless of the pad manufacturer, and you can repeat this process if your brakes develop fade over time due to glazing.

http://www.blackmagicbrakes.com/Brake_Pad_Break-in.html

 

[xpedition xj-eh!]

I've done the same conversion (including proportioning valve) and my fronts lock up before my rear in all cases (before and after zj disc conversion). The difference in brake bias is much less noticeable on pavement however. Which leads me to ask: What kind of surface are you testing you're brakes on?

I'm no brake expert but I'll throw out this thought: If on gravel, then you will notice the fronts locking up long before the rear (as mine do). If on pavement, then the tires will have more traction, and "front brake dive" pushing the front tires down (and simultaneously less weight on the rear)... which is perhaps why I notice better brake balance on pavement??

 

[Built2Flex]

An adjustable proportioning valve may be the fix to eliminate spooky braking in the wet.

This.

 

[sunburned]

I need to do something about mine too. Might just need to bleed them again or do something with the prop valve. I was driving on a slight down hill on solid ice and when I hit the brakes, the fronts skidded and the rears still rotated. Meaning my rear brakes are doing nothing.

Oh, and I've seen a military test video of a humvee doing a high speed run and locking up the rear brakes before the fronts. Multiple rollovers ensued.

 

[XJLI]

Fronts are supposed to lock first, that was mentioned already...

 

[JPs01XJ]

I changed my brakes with the discs from a Liberty about a month ago, and it has been awesome! Like everyone else said, you don't want any tire to ever lock-up.

Double-check your bleeding procedure, too. If you're working alone and are bleeding by only pushing on the brake pedal, it needs to be held down at full stroke before tightening the bleed valve. If not, it pulls air back in the caliper and defeats the purpose.

 

[Devilfrog]

I need to do something about mine too. Might just need to bleed them again or do something with the prop valve. I was driving on a slight down hill on solid ice and when I hit the brakes, the fronts skidded and the rears still rotated. Meaning my rear brakes are doing nothing.

Oh, and I've seen a military test video of a humvee doing a high speed run and locking up the rear brakes before the fronts. Multiple rollovers ensued.

that is the designed intention ... this way the rear wheels provide stability to the vehicle preventing an immediate spin ... wanna guess why you lose turning ability when the fronts lock up on an icy turn? The solution ... release the brakes and re-apply with less force, release and re-apply brakes in a quick pumping motion in order to slow yet prevent lock up of the front brakes. Sounds like ABS huh?
ABS was designed to take driver error out of the braking equation for this reason (most people just jam on the brakes in a panic situation hoping the skidding tires will slow them).

With a very generalized 60/40 standard front brake bias the front brakes should always lock up long before the rears ... wanna guess why the rear brakes always last ten times longer than the fronts?

 

[sunburned]

that is the designed intention ... this way the rear wheels provide stability to the vehicle preventing an immediate spin ... wanna guess why you lose turning ability when the fronts lock up on an icy turn? The solution ... release the brakes and re-apply with less force, release and re-apply brakes in a quick pumping motion in order to slow yet prevent lock up of the front brakes. Sounds like ABS huh?
ABS was designed to take driver error out of the braking equation for this reason (most people just jam on the brakes in a panic situation hoping the skidding tires will slow them).

With a very generalized 60/40 standard front brake bias the front brakes should always lock up long before the rears ... wanna guess why the rear brakes always last ten times longer than the fronts?

Uh, thanks dude... but I already went through all that in my vehicle dynamics course in engineering school... where I saw the video I mentioned.

 

[pokeyxj]

Thanks to everyone who has answered my post. Yes I was testing on gravel and yes I agree that there is better control with the fronts locking first. I bled the brakes with my son (22) starting at the RR, LR RF, LF. 2nd bleed with a vacumm bleeder. The pedal is firm, does not "pump up". I am driving it and it seems to be getting a little better. I'll let you know. Thanks again.

 

[Devilfrog]

Uh, thanks dude... but I already went through all that in my vehicle dynamics course in engineering school... where I saw the video I mentioned.

That's great then ... you're welcome ... and you should have already realized why your back brakes didn't lock up, and why it's not a problem, but by design.

 

[sunburned]

That's great then ... you're welcome ... and you should have already realized why your back brakes didn't lock up, and why it's not a problem, but by design.

You must not have read my whole post. I was driving on ice and they didn't stop the wheel from spinning. Meaning the rear brakes aren't even working. If I try to back down a steep hill and hit the brakes, my front wheels lock up and slide down while the rears do nothing. This is a problem, not by design.

 

[Vanimal]

If I try to back down a steep hill and hit the brakes, my front wheels lock up and slide down while the rears do nothing. This is a problem, not by design.

that's because your biggest brakes are still in the front when going in reverse. the front end unloads making it more likely to skid.
not saying your brakes are working right, but that's a pretty poor way to check them.

 

[XJLI]

that's because your biggest brakes are still in the front when going in reverse. the front end unloads making it more likely to skid.
not saying your brakes are working right, but that's a pretty poor way to check them.

X2 yo

 

[Devilfrog]

I didn't want to be "that guy" anymore so I waited, knowing someone else would also state the obvious soon enough.
If you don't think they are working, jack it up, put it on stands and have someone stand on the brakes while you attempt to turn the wheel ... while you're at it pull the wheel and check the rotors for grease/oil and the pads for wear.