Reality Check Rear Braking Power

Just so everyone is clear, there is no actual proportioning valve in a XJ. Google search "xj cherokee proportioning valve site:comancheclub.com", go to the Images tab and you'll find one cut open. The other is from an MJ with a cool little rear bias "proportioning" valve based on how loaded down the rear end was. You will not find a proportioning valve. Other than the shuttle valve for the warning light, there is zero connectivity between the front and rear circuits. By all indications, it's a metering valve, setting a minimum pressure that must be achieved before allowing the fluid to pass. By all means, prove me wrong, I'd like to know as much as anyone else. It looks more like a residual pressure valve, but the fact that the only difference with the ZJ is a stiffer spring leads me to believe otherwise. More residual pressure is not what you want with discs.

You'll see that the spring cavity will trap air; I'd bet money this is where your problem is. There's a bleeder built into the cap; you can bleed the rears from here to eternity and you won't get the air out otherwise. The front is generating full pressure, keeping the MC from stroking any further and it's compressing air in the rear instead of applying pressure at the pads. This is the safety mechanism against rear brake failure in action. In the case of front brake failure, the shuttle valve shifts to the left, opening the passage that bypasses the valve and the rear does the best it can...

Unless you have different front calipers, the LAST THING you want to do is increase the MC bore diameter without also swapping in the matching brake booster. If you want to increase performance with stock front calipers you need to decrease MC bore diameter, sacrificing pedal stroke and creating "softness" to the point that they get soft-soft-soft-lock em up because you're generating really high pressure but only once you've stroked it a good ways to engage the pads. If you've done the WJ brake swap or have tons, you need to do the math.

Neglected to mention that you can remove the gasket and achieve full rear braking potential. Downside is the increased chance the rear will lock up before the front; ZJ has stiffer spring to reduce the likelihood. Vehicle dependent; you have to try it. Good thing is, you can swap components without bleeding the rears...

Desert4x4 said:

Do you think the removal of the proportioning valve or the different master cylinder made the most difference?

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I had the E350 MC lying around for a different project. I decided to just use it on the XJ. It was not the easiest retrofit, almost bailed on the project halfway through. Figured try no proportioning valve to start as that would require extra work for a ridiculous project, I think I had a WJ valve I was going to use. Anyway, after testing without a valve both on road and on the trail I left it alone as it worked SO well. That being said, the MC was the biggest system change due to how much fluid it pushes. The valve is the fine tuning. I'm sure if I put a valve on the system it would be just a bit better.

No matter what changes you make, the MC will make the most difference. A new master + booster will be even better.

I'll take some pictures tomorrow to so you how it (barely) fits.

--Matt

I have the TJ rear discs, with XJ booster/mc, and stock prop valve. I pulled a bunch of prop blocks and valves from various junkyard jeeps and they were all the same. My braking performance is perfectly fine with 35s. The biggest problems I had were all related to air in the lines from the conversion, took a lot of effort to get the system fully bled.

ehall said:

I have the TJ rear discs, with XJ booster/mc, and stock prop valve. I pulled a bunch of prop blocks and valves from various junkyard jeeps and they were all the same. My braking performance is perfectly fine with 35s. The biggest problems I had were all related to air in the lines from the conversion, took a lot of effort to get the system fully bled.

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With respect sir, could you define "perfectly fine". Can you lock the front tires on dry payment in a panic stop? Also, could you describe your best bleeding method/procedure? Did you bleed at the proportioning valve?


Thanks a lot!

Desert4x4 said:

With respect sir, could you define "perfectly fine". Can you lock the front tires on dry payment in a panic stop? Also, could you describe your best bleeding method/procedure? Did you bleed at the proportioning valve?


Thanks a lot!

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Even better I can panic stop without locking the wheels. Seriously my trail beater stops as well as my DD XJ, and the trail XJ weighs 5k pounds with 35 inch tires vs stock weight and tires on my DD.

It's hard to bleed everything. Pressure bleeding through the reservoir is best if you have that option. Positive pressure and crack the various bleeders and flare nuts as needed.

ehall said:

Even better I can panic stop without locking the wheels. Seriously my trail beater stops as well as my DD XJ, and the trail XJ weighs 5k pounds with 35 inch tires vs stock weight and tires on my DD.

It's hard to bleed everything. Pressure bleeding through the reservoir is best if you have that option. Positive pressure and crack the various bleeders and flare nuts as needed.

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So "even better" meaning your maximum braking power won't lock the front tries? I agree as you implied, that this is a safer way to stop. Thanks.

Brakes slow the wheel spinning, tires stop the vehicle moving. If you lock up the wheels then you have overrun the holding power of the tires. Often more about the tires than the brakes.