Rear disk setup questions

[Matt Pascoe]

Here?s the setup:

Original '89 YJ MC and booster
Original '89 proportion valve un-modified
New front '95YJ calipers
New front CJ rotors
New rear Chevy Monte Carlo calipers
New rear Chevy truck vented rotors
35" tires

The problems, pedal is softer than before (with rear drums), rear brakes lock up first (cant get fronts to lock up), rear pads seem to maintain residual pressure on the disks.

I know these are all common rear disk swap problems, but all of my searches have found a real mix of opinions and fixes, especially going back a few years.

Here?s what I found:
"remove o-ring from proportioning valve"
"install a wilewood proportioning valve to adjust rear bias"
"install a different residual valve"
"install a Chevy van master cylinder"

I don?t want to chase my tail with band aids, what would be the appropriate fix?

Thanks
Matt

[Matt Pascoe]

Just found this:
1969 Corvette 350 with front and rear power disk brakes master cylinder bolts right up the the CJ/YJ/TJ booster and has the correct flow for rear disk. ~$30 at Autozone

If you have 1 ton rear drums (NOT DISK) the Ford E350 van MC offers the correct flow.

[blkTJ]

does this mean your finished with that flaring tool???

[Matt Pascoe]

Sure am (I hope ) You next in line for it? If so (and if Blaine gives it the OK) I'll drop it in the mail to you.

[blkTJ]

I'll be heading down to blaine's next weekend(I think). Just get it back to him when you can.

[John]

Once I finally finish up on my current axle project I'll try pulling the o-ring to drop the residual pressure to the rear. But I'm very interested in how yours comes out.

I'll have front and rear WJ calipers once I'm done. 12" rotors front and 11" rear.

[Matt Pascoe]

I'll try pulling the o-ring to drop the residual pressure to the rear.

Where exactly is this o-ring I keep hearing about? I know its in the combo valve under the master, is it behind that black cap on the end?

Any trick to removing it? Do you have to bench bleed the master afterwards?

[John]

Originally posted by Matt Pascoe
Where exactly is this o-ring I keep hearing about? I know its in the combo valve under the master, is it behind that black cap on the end?

Any trick to removing it? Do you have to bench bleed the master afterwards?

IIRC that's where it's at. Just pull the cap, yank the o-ring, and put it back together.
As long as the master doesn't drain all the way I wouldn't bench bleed it.

If you try it let me know.

[Matt Pascoe]

I just spoke with Van (Vanco PBS) and he confirmed what others had told me. He STRONGLY suggests that removing the o-ring from the combo valve is a bad idea.

He also said that the problem I'm experiencing is caused by the stock combo valve. It is designed to build up ~200 psi to the rear before it gives ANY pressure to the front.

He said the first step is to get a combo valve that is made for 4 wheel disk - ZJ combo valve

Second step (if the ZJ valve doesn?t solve all of the rear bias problems) is to install an adjustable proportioning valve to the rear.

Now the brake bias should be correct (fronts lock up before the rear) so you can address the lack of power (if its an issue). Van said that hydro boost is overkill for 35" tires and suggests his upgraded master/booster package. It?s a Corvette 1-1/8" master designed for front and rear disk mated to his higher pressure booster.

Thanks Blaine for getting me pointed in the right direction

[Robert J. Yates]

Van is a good guy but I am going to disagree with one of his staements - hydroboost is most certainly not overkill for 35's.

[speaceman]

Yeah, I agree with that also.

I had hydroboost on the ZJ with 33s (which I"d seat of my pants guestimate to be about the same as a TJ with 35s due to the weight difference between the two vehicles) and I'd say that I was right at the acceptible performance limits for safe road driving and safe stops on streets when dealing with other vehicles and traffic.

[Matt Pascoe]

I got lucky yesterday. The first junkyard that I went to had a '93 ZJ with the combo valve still intact.

It is a direct bolt-in, really easy.

The pedal feels great now, but the rear bias is slightly off.

On a dirt road, doing about 25 MPH I can lock up all 4, the rear locks up slightly before the front.

On pavement, same test, it stops great but the rears lock up just before coming to a complete stop. The fronts don't lock up.

I guess the next step is to try an adjustable prop valve.

I took apart the old combo valve (just to see how it works) and it was full of black sludge I found the same sludge in my old front calipers when I replaced them. I wonder how much of that crap is in my original master

[John]

So did you find the o-ring in question?

[Matt Pascoe]

Originally posted by John
So did you find the o-ring in question?

Not sure...

When you remove that cap with the rubber end, theres a piston that you can slide out. On the inside end of that shaft is a flat rubber washer with grooves. I think that flat washer must be what people are removing.

[Stu Olson]

correct....that would be it



Interesting to hear that your ZJ valve was a direct bolt in.

When I did mine several years ago, I had to add a "T" fitting to catch the other brake line.

[mrblaine]

Originally posted by Matt Pascoe
I got lucky yesterday. The first junkyard that I went to had a '93 ZJ with the combo valve still intact.

It is a direct bolt-in, really easy.

The pedal feels great now, but the rear bias is slightly off.

On a dirt road, doing about 25 MPH I can lock up all 4, the rear locks up slightly before the front.

On pavement, same test, it stops great but the rears lock up just before coming to a complete stop. The fronts don't lock up.

I guess the next step is to try an adjustable prop valve.

I took apart the old combo valve (just to see how it works) and it was full of black sludge I found the same sludge in my old front calipers when I replaced them. I wonder how much of that crap is in my original master

Did you check to see if that particular ZJ had the rear disc option? 93-95 rear discs were an option with drums being standard.

[Robert J. Yates]

You can also stagger your tire pressure a bit to fine tune your bias. I run the rears a bit higher than my fronts for that exact reason.

[Matt Pascoe]

Stu, mine looks just like that except the bracket is slightly diferent. You know that cap that is on the opposite end from the rubber cap? On my YJ valve that cap had a fitting on it for the second front brake line so I just swapped the YJ cap to the ZJ valve. I also completely removed the bracketry.

Did you check to see if that particular ZJ had the rear disc option? 93-95 rear discs were an option with drums being standard.

You know... its these little nuggets of info that I allways find out about too late From what was left of this ZJ (no rear axle) all I know for sure is it had ABS. I suppose I could go back and get the VIN and research what brakes it came with. Any other way to tell if I have a disk/disk combo valve?

Robert, I'll play around with the tire pressure and see if that helps

Thanks
MP

[Matt Pascoe]

Stu, mine looks just like that except the bracket is slightly diferent. You know that cap that is on the opposite end from the rubber cap? On my YJ valve that cap had a fitting on it for the second front brake line so I just swapped the YJ cap to the ZJ valve. I also completely removed the bracketry.

Did you check to see if that particular ZJ had the rear disc option? 93-95 rear discs were an option with drums being standard.

You know... its these little nuggets of info that I allways find out about too late From what was left of this ZJ (no rear axle) all I know for sure is it had ABS. I suppose I could go back and get the VIN and research what brakes it came with. Any other way to tell if I have a disk/disk combo valve?

Robert, I'll play around with the tire pressure and see if that helps

Thanks
MP

[speaceman]

Chances are that you hav e a drum prop valve unless the ZJ you pulled it off of was a limited, and even then I'm not sure.

Most of the older Zjs I've seen had drums on them.

[Robert J. Yates]

Originally posted by speaceman
Most of the older Zjs I've seen had drums on them.

Except for yours A piece of your old rig still lives under mine

[Robert J. Yates]

Originally posted by speaceman
Most of the older Zjs I've seen had drums on them.

Except for yours A piece of your old rig still lives under mine

[speaceman]

LOL.. i forgot about that.

[Chris L]

Witch piece would that be, cause I know were the old axles went

[speaceman]

This woudl be a piece from the original old axles, not the new old axles.

[VancoPBS]

Originally posted by Robert J. Yates
Van is a good guy but I am going to disagree with one of his staements - hydroboost is most certainly not overkill for 35's.

Hydroboost is not overkill for 35s. May be overkill with 35s and stock brakes, a duel diaphram booster works fine then. If you have 4 wheel disc, D44s or bigger I'd go hydroboost.

Overkill meaning, no need to spend the money when something cheaper would work. If it was up to me every Off-Roader would be running the hydroboost;-)

OK, I have swapped to rear discs (stock Rubicon discs) on my 97 as well. My brake system is otherwise stock, with the exception of 98 ZJ MC. I can lock up the fronts, but the rears do not lock up, unless I'm on ice or sand, no matter how hard I push on the pedal. What do I do?

[Jerry Bransford]

Originally posted by dennisuello
OK, I have swapped to rear discs (stock Rubicon discs) on my 97 as well. My brake system is otherwise stock, with the exception of 98 ZJ MC. I can lock up the fronts, but the rears do not lock up, unless I'm on ice or sand, no matter how hard I push on the pedal. What do I do?

With 35" tires & a rear disk brake conversion, I wish I could lock up even my front brakes!

[Stu Olson]

Right off the top of my head, I would say you could do the happy dance and call your project a success.

Since your rears are not locking up, the chances of your rear end trying to swing around and trade places with your front end is pretty much eliminated....and that would be a good thing for sure.

Given that the majority of your stopping power is on the two front tires, I think you are good to go.