Brake Master Cylinder Upgrade - JeepForum.com
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post #1 of 9 Old 11-14-2019, 12:01 AM Thread Starter
osgruber327
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Brake Master Cylinder Upgrade

Evening all,
I just ran across a you tube of a guy replacing the brake master cylinder in a 96 xj with a master cylinder from a 2001 Dodge Durango. As he explained, the Durango master was about 20 percent larger and pushed more fluid per stroke.
Does anyone know if this would work on a 93 zj? Has anyone done this? I recently came accross a steering gear upgrade that used a Durango steering box and pump. Is this common knowledge?
Thanks for all your help and advice in advance.

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post #2 of 9 Old 11-14-2019, 02:57 AM
Wolfeman1992
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I believe it does work on both

At some point the zjs change to a completely different steering box so the bolt pattern changes, never heard of changing the pump, I would check the mounting bolt patterns if there the same the swap should work

As for the booster I don't know about Durango but one from a later zj is a bolt in upgrade I believe you may need to swap the rod that goes to the pedal

94 ZJ 4.5" on 31s
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post #3 of 9 Old 11-14-2019, 08:13 AM
zjosh93
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Not sure how that master cylinder would be an upgrade on a stock ZJ. If you swap in a bigger master cylinder without changing anything else you'll just get a shorter, harder pedal and less brake force. Typically you'd swap in a bigger master cylinder to match bigger calipers so that your pedal pressure remains the same.
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post #4 of 9 Old 11-14-2019, 08:20 AM
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Quote:
Originally Posted by zjosh93 View Post
Not sure how that master cylinder would be an upgrade on a stock ZJ. If you swap in a bigger master cylinder without changing anything else you'll just get a shorter, harder pedal and less brake force. Typically you'd swap in a bigger master cylinder to match bigger calipers so that your pedal pressure remains the same.
True most upgrade the booster when doing the wj knuckle swap

94 ZJ 4.5" on 31s
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post #5 of 9 Old 11-14-2019, 03:22 PM Thread Starter
osgruber327
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Thanks for the input all.....
I am a bit confused about something though,
If the Durango master cylinder is 20 percent bigger, wouldn't you get more force with less effort at the pedal? 20 percent increase with the same pedal travel - wouldn't that correspond to a proportional increase in braking force at the caliper? Thanks again!

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post #6 of 9 Old 11-14-2019, 03:52 PM
zjosh93
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Hydraulics are a little unintuitive that way. Force on the slave cylinder is equal to the area of the slave divided by the area of the master. So to get more force you need a smaller master diameter. But because it's smaller in diameter it needs to move farther for a given volume of fluid. That's why the tiny piston on a jack can lift your car but you gotta pump it a bunch. If you get bigger pistons on the calipers your pedal will get longer and softer because bigger caliper pistons have more area and need more fluid volume.
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post #7 of 9 Old 11-14-2019, 04:07 PM
zjosh93
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In the case of using the Durango 1 1/8" master on the ZJ:

Original master is 15/16" so it has an area of 0.69 square inches. The stock calipers are 66 mm or 2.6" for an area of 5.30". So the effective ratio is 7.7. Ignoring the pedal ratio and the booster, each pound of force on the master becomes 7.7 lbs at the caliper.

The Durango master is 1 1/8" so it has an area of 0.99 square inches. With the stock calipers that's a ratio of 5.3. You'd lose about 31% of your braking force.

FWIW you don't need to do all the above to get there. 20% bigger is a multiplier of 1.2, 1.2 squared is 1.44, the inverse of 1.44 is 0.69. So you'd have 69% of the original braking (reduction of 31%).
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post #8 of 9 Old 11-14-2019, 04:46 PM Thread Starter
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You all are great.
I'm glad I found this resource.

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post #9 of 9 Old 11-14-2019, 04:57 PM
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Quote:
Originally Posted by zjosh93 View Post
In the case of using the Durango 1 1/8" master on the ZJ:

Original master is 15/16" so it has an area of 0.69 square inches. The stock calipers are 66 mm or 2.6" for an area of 5.30". So the effective ratio is 7.7. Ignoring the pedal ratio and the booster, each pound of force on the master becomes 7.7 lbs at the caliper.

The Durango master is 1 1/8" so it has an area of 0.99 square inches. With the stock calipers that's a ratio of 5.3. You'd lose about 31% of your braking force.

FWIW you don't need to do all the above to get there. 20% bigger is a multiplier of 1.2, 1.2 squared is 1.44, the inverse of 1.44 is 0.69. So you'd have 69% of the original braking (reduction of 31%).
Holy ****

94 ZJ 4.5" on 31s
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