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post #1 of (permalink) Old 12-12-2005, 09:50 PM Thread Starter
 
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brake proportioning valve.

since i installed my ford 8.8 and got disk brakes in the rear mybrake pedal has felt very mushy and the rear locks up before the front. (actually i cant even get the front to lock any more) i heard that a proportioning valve can be used from a zj to fix this but i was wondering if one could be used from a rubicon too since it comes with 4 wheel disks. i figured if its a differnt vavle it would be better than a zj valve because of the weight differnce. or does the rubicon just use the same valve and some other means of dealing with the rear brakes?
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post #2 of (permalink) Old 12-12-2005, 10:20 PM
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Re: brake proportioning valve.

Proportioning valves are used to balance the different characteristics between drum and disc brakes, since they start operating at different pressures. Which end do you have drums on?
Discs start braking with a small amount of pressure, drums take more to start, but once engaged they take less pressure.

No drums?

Then the balance front to rear is determined by the diameters of the caliper's pistons and the master's pistons.

Look up the MC bore size of what the front is supposed to use determined by the caliper bore size, then look up the size required for the rear. Then find a MC that has those sizes front to rear. That's the trouble with arbitrarily mixing and matching unrelated parts. Just because it bolts up, doesn't mean it'll work correctly.
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post #3 of (permalink) Old 12-14-2005, 07:30 PM
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Re: brake proportioning valve.

Mine works perfectly without an adjustable proportioning valve.

Try bleeding the fronts?
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post #4 of (permalink) Old 12-15-2005, 12:24 PM
 
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Re: brake proportioning valve.

they a checkvalve in the master clyinder thats has to be removed.
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post #5 of (permalink) Old 12-15-2005, 12:35 PM Thread Starter
 
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Re: brake proportioning valve.

skellator, did u do the remove the o-ring in the prop. valve? i hear this trick helped with the disk conversion but i could not find an o ring to remove. maybe i didnt look hard enough?

brown bag,
can u tell me more about this. i have never heard of anything with the master cylinder before.
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post #6 of (permalink) Old 12-15-2005, 01:56 PM
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Re: brake proportioning valve.

The O ring is in the differential valve part of the combination/proportioning valve. Removing it disables the proportioning valve, just like you didn't have one.

Master cylinders designed for drum brakes have a check valve in the outlet. Unscrew the brake line at the MC - look up inside the outlet. If you see a brass object in there, use a sheet metal screw to remove it.

But - that check valve isn't your trouble as you described. If it's still in there it will cause that end to lock up and stay locked.

It's purpose is to keep a slight pressure in the brake line for the drum's wheel cylinders. If that pressure is in the disc's line, the the discs stay applied, can't release. Remember, drum type shoes are pulled away from the drums with springs. Disc pads are pushed back only by the slight runout of the discs.

Unfortunately many rebuilders don't understand the purpose of the check valve. They often remove them on all rebuilds. Then if you use that MC with drum brakes, it acts like you cannot get all the air out.
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post #7 of (permalink) Old 12-16-2005, 05:05 PM
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Re: brake proportioning valve.

[ QUOTE ]
skellator, did u do the remove the o-ring in the prop. valve?


[/ QUOTE ]


No. Never did anything but put in my 8.8 and bleed the system. The stock proportioning valve and master cylinder are a few things on my Jeep that hasn't been replaced or modified.
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post #8 of (permalink) Old 12-25-2005, 10:38 PM
 
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Re: brake proportioning valve.

I DID remove the o-ring on mine. It works great now with good balance front-rear.
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