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Discussion Starter #1
My brakes suck. I have a '93 XJ on 34's with the stock 30 front and rear disks from a ZJ on an 8.25. I removed the o-ring in the proportioning valve when I installed the rear disks. Last week I swapped in a master cyliner from a one ton E-350 and now my brakes suck slightly more. What should I try next? I was thinking about swapping in a dual diaphram booster and going back to a smaller MC which would increase line pressure but after pricing parts I'm considering other alternatives to imporve braking performance. What exactly would removing the proportioning vavle accomplish if anything? What does it do beside bias braking front/rear?
 

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Wes in TN said:
What exactly would removing the proportioning vavle accomplish if anything?
I removed mine for more braking bias in the rear. Take the vehicle to gravel, try and lock up the tires. If the rear lock up, you're getting enough bias. If not, I'd consider removing the prop valve.

Bryan
 

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Discussion Starter #3
Okay, I'll make the question more clear, does the proportioning valve restrict the total system pressure in any way or does it only limit pressure to the rear of the system? Will removing it improve my braking at all?
 

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full presure go's to the front, the prop valve limits the rear.

id say go back to a smaller mc. the 1 ton cyl you put on has to big of a piston , and to much volume.
 

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Wes,
Like Oxjockey told you, full pressure to the front and the proportioning valve limits pressure to the rear.

The proportioning valve limits the amout of fluid to the rear when the vehicle is empty, so you wont lock up the rear brakes and lose control. It also allows more fluid to the rear when you are carrying more weight so the fronts arent doing all of the stopping and you can maintain control. If you remove the valve you will get full pressure to the rear whether your loaded or empty. Not a big deal if you dont wait till the last second to brake and its wet outside.

Good luck!!

JJ
 

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the prop valve in XJ's doens't increase pressure related to amount of weight in the truck.

now the commanche's did have a weight sensitive prop valve that would increase rear braking with more load in the bed.
 

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xj4rocks said:
the prop valve in XJ's doens't increase pressure related to amount of weight in the truck.

now the commanche's did have a weight sensitive prop valve that would increase rear braking with more load in the bed.
Most pick ups have the weight sensing prop valves, no need in an XJ.
 

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I would not put a smaller MC back on. The proportioning valve only holds pressure off to the rear brakes.
If you have worse brakes then when you started you might have air in the system. I have a lot of customers put the rear calipers on the wrong side. I know it seems elementary but check to make sure the bleeder is pointing up. They cuss me for everything I am worth then find out they put them on wrong.
You may also not be pushing the MC far enough. I know on a YJ the e350 MC sawp is direct with a little grinding. The push rod may have to be adjusted on an XJ. I dont know if they use the same booster setup(kinda doubt it). That gives you a few things to look at. Rocky
 

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The YJ and XJ do use the same booster.

Make sure your Bleeders are up!

Bleed it again.

Test. If still no improvement, remove the Prop Valve.

As you know from our PM's and my report I am running with no prop and it is just right for my Q78's.

Later.
 

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Discussion Starter #10
Bleeders are all up. YJ and XJ boosters are the same, I even checked the new master vs the original XJ master before I bolted it up to make sure the rod lenght didn't need adjusting, it didn't. I bled the system twice, the pedal is firm, there just isn't enough stopping power there. I'm going to bleed it again just to make sure before I go swapping parts again. After doing some research on combination vavles I can't see why removing it would affect the line pressure any so I'm going to leave it. Here is a site that describes the function of a combination valve fairly well. The larger piston in the E-350 moves a larger volume of fluid but at the expense of line pressure. I need more line pressure and the only way to get that is with a larger booster. I guess that is my next investment because I can't get anywhere near locking these things up at any speed.
 

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I've done the swap in my YJ and the pedal feel is better. I had bigger brakes front and back and the pedal would almost hit the floor before. I can lock the fronts up, but not the rears. I'll be doing the prop valve trick as soon as I've got time and the wife will sit in the Jeep and pump the brakes.
Let me know what bigger booster you find that works. I need one too.
 

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Brake Masters

Master cylinders for disc/drum and disc/disc are different. Drum brake cylinders have a check valve that does not release all of the fluid when the brakes are released. The check valve holds pressure on the lines to keep the cups in the wheel cylinders expanded to help prevent leakage. This check valve on disc brakes will cause the pads, which have no springs to pull them back, to remain against the rotors. Not enough so that you cannot move or anything, but enough to cause excessive wear and a jerking as you roll to a stop. One of the most common masters to use is from 79-80 Corvettes, which has four wheel disc. They have a 1 1/8" cylinder and work fine. Remember, that the cylinders hook up what would be called backwards the line to the front brakes comes from the outlet closest the firewall. The rear brakes come off the front outlet. This is also important to the way the calipers pressurize.
 

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Brake cylinde

I do not know of any normal truck that has four wheel disc. This really is an important consideration. Sure others may seem to work, but they will not be right. The check valve I mentioned is one of the reasons. The other is that with the proper cylinder, the front brakes get more pressure first. The rear brakes do not receive an equal amount. This prevents the rear brakes from locking up first which could send you looping around. I know, I know. That is what the proportioning valve is for. But, use the correct cylinder and you most likely will not need the valve. If it is a cylinder for disc/drum, then at the very least you will have to remove the check valve.
 
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