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post #1 of (permalink) Old 05-28-2002, 06:03 AM Thread Starter
 
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4whl disc brakes on YJ, need larger Master Cyl?

I just finished the 8.8" swap (it's drivable but perches aren't welded yet) and it stops about as good as it use to. I bled the brakes real good, still nothing respectable. I think I need a larger master cylinder with a bigger bore and reservoir. Any ideas on a good/easy swap. Something late model is preferable (easier to find at the bone yard I go to). Thanks, big ed.
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post #2 of (permalink) Old 05-28-2002, 06:56 AM
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Re: 4whl disc brakes on YJ, need larger Master Cyl?

Usually disk brake master cylinders have a larger reservoir because brake fluid provides the wear adjustment. As the pads wear the caliper pistons move out farther and stay there. That requires more fluid to fill the caliper.

Disk brakes usually require a smaller piston in the master cylinder because they inherently take more pressure to activate, but don't need the large volume of fluid that drum brakes do just to move the shoes out to the drums.

But that is affected by the pedal linkage ratio, the piston area in the caliper, the proportioning valve, and the ratio of front to rear braking you need on your vehicle.

You need to do some research on your entire braking system and figure out what master cylinder you need. You might not help yourself just by the swapping master cylinder for one you think might work better.

A couple more observations; rear brakes don't do much of the braking anyway, so don't expect a big improvement in stopping performance.

Don't drive with those perches unwelded! If the axel slips you'll be in a heap o' trouble.
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post #3 of (permalink) Old 05-28-2002, 08:48 AM
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Re: 4whl disc brakes on YJ, need larger Master Cyl?

First, weld those perches before driving!

Next, the factory proportioning valve on a YJ sends less pressure to the rear brakes cuz they'd blow if got the same pressure as the fronts. Now that you have discs in there you can disable the prop. valve and enjoy better breaking. I did this mod and now can lock up my wheels. Remove the nut that is on the front of the prop. valve manifold then pull the spring and plunger out with needle nose pliers. Remove the rubber o-ring from the end of the plunger and reinstall everything back the way it was. Voila! Do this before you spend on anything else. As the next step you could install a new stock master cylinder as the old ones many times develop a slow leak and won't supply full pressure. You can tell this is happening when if holding the pedal it'll slowly sink to the floor.
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post #4 of (permalink) Old 05-28-2002, 11:39 AM Thread Starter
 
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Re: 4whl disc brakes on YJ, need larger Master Cyl?

Trust me, I'm not getting on it with the perches unwelded! I just wanted to get up to 50mph or so and check for vibration before I weld. I'm going to change my angle a little, then drive it one more time, then weld hopefully.

I didn't even know that I had a proportioning valve on the YJ, will definitely check that out first, sounds like it will help. I'm not looking to lock them up but hope to at least come close.
Thanks for the tips guys, will keep you posted,
big ed
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post #5 of (permalink) Old 05-28-2002, 12:47 PM
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Re: 4whl disc brakes on YJ, need larger Master Cyl?

Apparently you missed this one the last few times I posted it. This is what happens when the perches brake their welds. Can you imagine how easy the axle will rotate with the axle ground smooth with smooth spring perches? Please don't drive your jeep again without welding the perches. At the least, run some 1/2" long beads at the four corners of each perch.

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