Rear disc conversion working too well
 

So last year I did the ZJ rear disc swap on my XJ. I notice the most difference on slick roads in the winter. I went from not being able to get the rear brakes to lock up at all with the drums to starting to lock up prematurely. Let me explain the issue a little more.


When I'm on dry pavement I don't have any problem, brakes work great. However when I get on slick surfaces and try to slow down the rear brakes are grabbing slightly harder/faster than the front. I'm not talking about slamming on the brakes either, just modulating speed down an icy hill, or slowing down slowly/normally. They don't grab hard enough to actually lock up and engage the ABS, but they will start to slip enough that I can feel the back end start to kick out going down a hill or if I slow down in a curve. Makes for a little pucker factor when you try to slow down a little on the expressway in rush hour traffic and the back end starts to break loose a little. :icon_eek:


I'd have to go try it to confirm my memory is correct but if I brake harder it problem seems to diminish or go away, like the front brakes kick in and start doing their job and things balance out. On ice the whole Jeep seems to break loose evenly and the ABS kicks in.


Anyone else run across this? I'm running more or less stock tires and I put in the ZJ proportioning valve when I did the swap. Prop valve shouldn't be doing anything under light braking anyway but I'm kind of baffled.


I've read about people swapping in WJ front brakes for added stopping power, but I guess I don't know how the ZJ front discs compare to the XJ. I could see needing the WJ fronts for even braking force if I had taken WJ rear brakes, but I grabbed ZJ hardware so that's an unknown.


Any ideas are appreciated. Oh, and I don't believe there to be any residual air in the front lines as I have a nice firm pedal.

Changing the discs shouldn't make much of a difference except for adding thermal mass and area to stay cooler, but that won't matter unless you're at the limit. Grippier pads would make a far bigger difference. Do you have fresh brake lines or SS braided lines in front?

I have the same problem and feel the same as you. The rear engage quicker so lock on slippery surface but when putting the brake pedal to the floor on the street all seems normal. I would get better performance pads on the front. Sadly I can't find anything but OEM for my 89 so plan to convert to later model knuckles.

Not sure, but will tell this anyway

has the proportioning valve been upgraded as well, to one used in a 4 disc vehicle.

I'm thinking that maybe most of the initial braking is going to the rear tires, and later to the front, causing the rear to start to slip prematuraly on icy roads.

Maybe the experts will chime in and correct me

OP stated he swapped to the ZJ which is proper for the swap.

I'd recommend more aggressive pads in the front. Do Hawk LTS fit your calipers?

Sounds like your front calipers need rebuilt. I had the same problem, turned out, my calipers were hanging up. Auto zone sells them for $22 a piece

Ahh I like it. Manly hug for you.

Hahaha don't forget, after you rebuild/replace the calipers, bleed the brakes properly and thoroughly. You could have 15" Brembo's and they still won't work right if there is air in the lines. Check all your rubber lines and fittings as well.

Did you change master cylinder ?

Change spring in prop valve yet ?

In your first post you mention ABS.
Did you put the ABS sensors back in place?
If not then then I think the ABS computer should be showing a fault light.
The ABS computer may not be seeing the "real" wheel speed and allowing the rears to lock up.
The whole point of ABS is to NOT lock up wheels so you continue to have control of the vehicle direction during hard braking.

heres the deal. could be an install issue (with prop valve or ABS crap), could be a rear brake issue, or could be a front brake issue. Hope that narrows it down!

If its a front issue, it could be your 15 year old front calipers are just tired and dont operate smoothly anymore. New calipers at advance auto for $20 each after core replacement, use a coupon code to bring them down to $15 each. Could also be the pads not being that great and not biting as well as the rear.

If its a rear issue, it could be crappy pads. I know Some cheap pads when wet will swell up enough to cause issues like you have.

First thing I would do is inspect the front calipers, and make sure they work smoothly and the slide pins are sliding without much resistance.

I'll try to answer the questions as best I can...


Front brakes are stock. I believe I changed the pad and turned the rotors soon after buying the Jeep a couple years ago. Still in great shape. The left caliper was replaced as well as the brake hose because it was sticking. Right one has not been changed. There is no pulling right or left when applying the brakes to indicate a sticking caliper in the front and I have a hard time thinking both the original and replacement would stick the same amount. Hubs do not heat up either. When I've had calipers stick in the past it's not a problem with them not engaging, it's a problem with them not releasing and dragging. BNJeepsta, how much were your calipers sticking, was it enough to heat up the hubs? I suppose it wouldn't hurt to check them and verify they are smoothly operating.


The guts of the proportioning valve were changed in their entirety (plunger, spring, cap) when I did the swap. ABS is all there and functional.


When I changed the brake hose on the front left side I also purchased one for the right but have yet to install it. I may try doing that when I get some time since I have it. It is not collapsed, but that does not mean it's not expanding a little which could plausibly create this symptom.


Master cylinder is OEM, I've had no need to get into it.


What defines a "grippier" pad? I usually just get the economy ceramic pads if memory serves me right.

Try changing the valve back to the old one. It SHOULD make the brakes a 70/30 ratio instead of 50/50.

Good luck.