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Old 07-01-2009, 06:50 PM   #1
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Spongy pedal after disc brake conversion

I have a 1994 yj. I converted the rear brakes to disc. Installed the proportioning valve from a 4 wheel disc grand cherokee, bleed the system 3 times and the pedal is still spongy the first time you push it. It is fine the second time, anyone have any ideas?
Thanks for any help.

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Old 07-01-2009, 08:41 PM   #2
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You might try bleeding the lines at the master cylinder.

Use the "have an assistant pump the pedal & hold it" method. Just be sure to use line wrenches & close it before they lift the pedal.

Then re-bleed the calipers.

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Old 07-01-2009, 09:57 PM   #3
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does the pedal drop to the floor with light pressure? problem with bleeding an older system is that the master cylinder doesnt normally see full compression but when bleeding the master gets pressed all the way in. Being old the piston inside the master may have a lip at normal operating stroke which can put little tears in lip seals inside master when pushed past lip. double pumping can help push seals against cylinder wall and help get some pressure but in emergency stop that wont cut it. also if it hasnt been flushed in a while you might have jelled fluid and its clogging ports that supply master. what you do here will put your safety on the line. bleed a few more times but at any moment the pedal goes to the floor replace the master.
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Old 07-01-2009, 10:03 PM   #4
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another thought is use a master from a 4 wheel disc jeep. rear disc pistons are bigger and require more fluid to move than wheel cylinders. 4wheel disc master might have a bigger piston inside.
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Old 07-02-2009, 07:41 PM   #5
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Quote:
Originally Posted by dooder View Post
another thought is use a master from a 4 wheel disc jeep. rear disc pistons are bigger and require more fluid to move than wheel cylinders. 4wheel disc master might have a bigger piston inside.
That's absolutely true. Some folks, doing the 8.8 axle/disc swap say they got good results using a master cylinder from a Mercury Marquis although I'm not sure which year.
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Old 07-07-2009, 01:26 AM   #6
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Quote:
Originally Posted by jsklenar View Post
I have a 1994 yj. I converted the rear brakes to disc. Installed the proportioning valve from a 4 wheel disc grand cherokee, bleed the system 3 times and the pedal is still spongy the first time you push it. It is fine the second time, anyone have any ideas?
Thanks for any help.
you have air in your system.

did you bench bleed the master?

did you bleed the combo/prop valve?

there is air somewhere... be prepared to run a couple quarts through the system.. =(
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Old 07-11-2009, 06:47 PM   #7
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Well I followed the instructions with the kit, the calipers were installed wrong. being left on the right and right on the left. This put the bleeders at the top. I installed them this way against my better judgement, swapped them with some info from another forum and now eveything works great. Stops much better than the old drums.

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