I just picked up a ZJ rear axle for a disc conversion on my XJ. My question is; am I going to need to also swap the proportioning valve from a ZJ? Or can I get away with using the XJs. I've read a couple write ups and neither said anything about needing to do so.

i have heard that "if you want rear disks then take the ZJ proportioning valve as well because disks require more fluid than drums to work properly, all the other people who dont take the proportioning valve are just the cool kids on the block who want disk brakes"

with that being said, i still have my XJ valve in but i feel that if i had the ZJ they would be better

The guy I bought the axle from had swapped in a D44 and was not selling the valve. I'll probably pick one up at local pick and pull or get a new one. Willys55 how much you looking to get for yours?

I didn't swap mine when I had my old axles. Drums require pressure all the time and disc don't so the disc will rub a bit but I only noticed it when I was driving beside a wall and could hear it.

Drum and disc require different pressures and volume of fluid. Swap in the ZJ prop valve opr you wont get proper breaking in the rear.

I'v had rear discs for 10 years, many years i ran the stock single diaphram booster and master with XJ prop valve o-ring removed. Few years ago i finally swapped in my dual diaphram booster/master and ZJ prop valve internals, what an improvement! No more nose diving and it stopped very well.

I thought I could just swap the prop valves. You swapped the internals? Guess I'll have to find a ZJs and take a look at it. Can I run this without the booster and master. ( just the dics and prop-valve)? I want to do this one time and the right way.

You already have the newer booster and master.

The internals are easier to swap than the whole assembly. Remove the endcap, spring, plunger and oring. There are differences in these parts but they fit inside the XJ housing.

Reinstall the ZJ stuff.. all of it.

what you need is the zj internals and two of the lines coming into the xj proportion valve needs to be enlarged by one size up. i just put a 8.8 in

actually, what i meant to say is that where the 4 brake lines come into the proportional valve, the orfices in the proportional valve itself, 2 are a size smaller by drill bit size and needs to be enlarged by only one drill bit size. My 1999 then stops perfect with stock tire sizes. (i ditched the abs system) sweet! soon i am going to do a complete install write up of the ford 8.8 rear due to their was too many conflicts of questions and answers on these websites.

Thanks guys, I can't wait to lose these drums.

I am trying to revive some old threads...

Over the decades, the general internet-knowledge consensus was that using a drum proportioning valve with disc will result in premature lockup. Well, I did the swap for the first time on any car in my life on my 1999 XJ and that is definitely not the case. In fact, no matter how much I stand on the pedal, the rears will not lock on a snow packed road. I do have the ZJ proportioning valve guts that I haven't installed yet...

I went with rear disc because I got the bug in my brain that suggested that it would help me a bit if I snapped an axle shaft. With drums, that axle shaft is walking right out of there. With disc, it'll stay in there. Parts will be boogered, but those are relatively easy to replace and I can limp to a more convenient place to call for a recovery.

So on my 98 xj I swapped to disc from liberty. I went to go test drive after bleeding and the pedal is super soft and smushy. I believe it's seaping at the booster but I wiped it off and will check it each day. Now I'm not sure if the rears are even engaging. The new rotors have score hash marks, i.e. like it's right out of the box. To stop the car I have to slowly tap the brake then almost hit the floor. So far not leaks in the lines.

In my experience, the most common cause of this is calipers installed left for right, a.k.a. upside down. The bleeders need to be at the top of the piston bore.

I am assuming the brakes worked normally before the brake swap and nothing else has been messed with.

Well I have looked at them and the bleeder is in fact above the brake line indicating its the top. Now I have a broken bleeder but it's closed and not leaking. Also I have a PV4 type gm disc/disc combination (proportioning) valve. At this point I'm thinking about a new master cylinder and a wildwood adjustable valve.