Tips for '06 Rubicon axles into a '01 TJ

[TJeeper]

It seems to me that the discs in the rear are more powerfull than the drums were, so it's easier to lock them up now. My wife locked them up coming down a fast wash a couple weeks ago.

Did the fronts lock up first?

The modification for the combo valve increases the pressure to the rear brakes, you don't want that.

If the rears are consistantly locking up before the fronts, you will need to plumb in an adustable proportioning valve. This is what I have in mine:

Wilwood proportioning valve

Patrick

[TJeeper]

The proportioning valve (in its OE configuration--in your case disk front/drum rear) will not allow the brakes to function properly unless you get extreamly lucky and end up with the same fluid requirements as the OE configuration.

Really? So you have experience with rear disc converisons?

FWIW I purchased a combo valve from an '03 Rubi, it was exactly the same as the combo valve that I have in my '99. They did have different part numbers, which is why I bought it. I have yet to see any evidence different combo valves were used in the various TJ models.

When I did my rear disc conversion with ZJ rear brakes, the original combo valve worked fine on my Jeep. I could only lock up the rear brakes at the end of a very hard stop- pedal to the floor. I could never lock up the fronts, so I upgraded to hydroboost, which is when the adjustable proportioning valve went into my Jeep.

[JpPassenger]

Did the fronts lock up first?

Patrick

I finally remembered to confirm it with my wife -- the rear locked up first.

I'm looking at getting a Rubi proportinal valve. I feel pretty confident that it will solve the issue.

We have definately opened the door for more damage to occur. The jeep would simply get hung up on the toughest obstaces (our members long ago quit telling her our jeep can't do "that" obstacle.) It was a good limit, I just had to replace tires for the first 4 years. Now, nothing stops her. In the brief time since the transformation: near rolls=4 (including a flop onto the side that righted itself.) mangled tailgate=1. creased rear corners=3. shredded softtop=1.

Some people set up college funds for their kids. We set up a maintenance fund for the Jeep.

[TJeeper]

I'm looking at getting a Rubi proportinal valve. I feel pretty confident that it will solve the issue.

It won't. I have one, it is the same as the one that came on my '99. The only way to reduce the pressure to the rear line is to install an adjustable proportioning valve like I said above. Stu has some pictures on his site how he did his.

[JpPassenger]

That's a pretty good writeup, Patrick. Thanks for sharing it.

I've had a few more people pointing out that an adjustable valve is needed, or the proper master cylinder for the rear disks. So I'll strike my previous "pretty confident" statement about a rubicon PV.

Thanks for the help.

[TJeeper]

or the proper master cylinder for the rear disks.

I am not sure what you mean by changing the MC to solve your problem. It won't, and if you think about it, it doesn't even make sense. All you need to do is reduce the pressure to the rears.

[JpPassenger]

I am not sure what you mean by changing the MC to solve your problem. It won't, and if you think about it, it doesn't even make sense. All you need to do is reduce the pressure to the rears.

I checked my factory manual and confirmed my '01 Master Cylynder has a primary circuit for the front brakes, and a secondary circuit for the rear. This redundancy is pretty common for all modern vehicles.

My brake guy was the first to point this out: a MC for discs/drum would have different piston sizes than a MC for a disc/disc system.

I have a pretty good working knowlege of hydraulics, and it does make sense.

Both pistons are on a common shaft, so they have the same stroke, so the bore size differs. If the drum circuit would require more pressure to apply the brakes than a disc, the bore size would be larger for the drums, supplying more fluid volume, which translates into more pressure. Of course, the caliper or slave cylinder sizes factor into all of this, but I don't have anything to disassemble and measure to do the math.

The real question: is there a difference between MCs or PV for disc/drum systems versus disc/disc systems. Of course they look the same, but a millimeter difference in bore diameter is all that's required.

I have no doubt that the adjustable PV will work, however I'd preffer to fix it with stock parts than special order stuff.

[TJeeper]

My brake guy was the first to point this out: a MC for discs/drum would have different piston sizes than a MC for a disc/disc system.

You are overthinking this. The MC is the same for all TJs, regardless of year or rear brakes. Also the same as ZJs and newer XJs.

[Drewminator]

No ABS on the '06 Rubicon's, either. Since we had the 30" wheel package with the D44, we didn't have ABS on our '01 either. But it is something to consider with the swap.

It seems to me that the discs in the rear are more powerfull than the drums were, so it's easier to lock them up now. My wife locked them up coming down a fast wash a couple weeks ago.

I will be the first to admit I don't know the answer, and am happy to hear from those with more experience. I believe the area of the disc piston is greater than the area of the slave cylinder pistons on the previous drum brakes. So the same psi will create more force across the larger area (and the brake pedal does requires more travel, corresponding to more fluid volume needed to move the piston.) I'm not certain how a proportioning valve helps after the initial application.

Are you sure there isnt ABS on the 06 Rubicon, because there is an ABS icon on my dashboard that said ABS?

[TJeeper]

Are you sure there isnt ABS on the 06 Rubicon, because there is an ABS icon on my dashboard that said ABS?

There is a full time 4WD icon on my dash, does that mean my 231 has a full time mode?

[Drewminator]

There is a full time 4WD icon on my dash, does that mean my 231 has a full time mode?

That’s odd. Wonder why they would put an icon on the dashboard if the functionality doesn’t exist. Oh well, thanks for pointing that out.

[goodtimes]

That’s odd. Wonder why they would put an icon on the dashboard if the functionality doesn’t exist. Oh well, thanks for pointing that out.

They may be using the same guage cluster in all the TJ's, regardless of them having ABS or not. Fewer line items to keep track of = lower cost.

[Drewminator]

They may be using the same guage cluster in all the TJ's, regardless of them having ABS or not. Fewer line items to keep track of = lower cost.

Oh, got it. Thanks for the information MR. goodtimes

[JpPassenger]

You are overthinking this.

Yeah, You aren't the first to level that accusation, and you surely won't be the last.

I do appreciate your tips.
Thanks,
Greg

[TJeeper]

They may be using the same guage cluster in all the TJ's, regardless of them having ABS or not. Fewer line items to keep track of = lower cost.

They also used them in the later XJs, which is what the full time 4WD icon is for.