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Unread 12-13-2013, 07:45 PM   #1
Jeep01AH
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Wj Master Brake Cylinder going bad?

I've done some reading and I think I've got it narrowed down, but I just want some input from everyone else.

Lately the Jeep has been sitting outside all night in sub zero temperatures for days because I'm keeping it at school. When I go to start it to go to work, there is no pressure behind the brake pedal. I would pump it for a minute and it would come back. Today when I was driving home, I didn't use the pedal for a while because I was on the highway for over two hours. When I finally came to a stop light, I had no pressure and wasn't able to stop. Luckily the light turned just before I went flying through. This is the first time this has happened while driving. I have no leaks and my fluid is full. Is this the master cylinder going out on me?

Any help is appreciated!

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Unread 12-13-2013, 07:54 PM   #2
WiLieR
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If you can pump the pedal and it will build pressure and work then the master cylinder is the one that is extremely likely the culprit. If it were my WJ I would replace the MC and go from there.

Check you soft hoses as well. Most people never think to replace those and they are 9-14 years old by now and most likely needing to be replaced.
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Unread 12-13-2013, 07:58 PM   #3
Jeep01AH
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Thanks for the reply!

I am looking into replacing the hoses with braided steel lines in the near future. I suspected the master cylinder though. Thanks for the help.
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Unread 12-13-2013, 09:07 PM   #4
michiganmaveric
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Get it changed ASAP.
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Unread 12-14-2013, 12:47 PM   #5
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+1 Been there done that....
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Unread 12-20-2013, 09:34 PM   #6
Jeep01AH
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I had a shop change out the master cylinder and I'm gonna assume they bled the brakes properly, but now I have a really low brake pedal. The brakes will not engage until about 2 inches from the floor. Does anybody have any ideas what could be causing this? Nothing is leaking and I have enough fluid. There is one pretty rusted spot on the hard line traveling to the rear axle. Could air be seeping in this way?
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Unread 12-21-2013, 04:33 AM   #7
MadderThanMax
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Brake Servo may not be operating as effectively as it should.. this would cause a drop in pedal..If it has a hose connection to it, with the engine off pull the hose out, It should *pop* if working properly if it hisses then its a fail and needs replaced...
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Unread 12-21-2013, 05:20 AM   #8
doityourselfMM
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What makes you assume that they bled the brakes correctly? There is a procedure for bleeding the master cylinder on the bench, prior to installation in the vehicle. And there is also the ABS system and bleeding procedure.

Not saying the shop didn't do the job correctly, just saying I wouldn't assume they did, if the pedal only begins to engage the brakes close to the floor.

Don't ignore that rear brake line. That rusted line is a common problem on WJs. I actually had that line explode on my 99. The pedal went to the floor and brake fluid squirted everywhere. If that line ruptures, you will have no brakes.



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Originally Posted by Jeep01AH View Post
I had a shop change out the master cylinder and I'm gonna assume they bled the brakes properly, but now I have a really low brake pedal. The brakes will not engage until about 2 inches from the floor. Does anybody have any ideas what could be causing this? Nothing is leaking and I have enough fluid. There is one pretty rusted spot on the hard line traveling to the rear axle. Could air be seeping in this way?
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Unread 12-21-2013, 11:43 AM   #9
Jeep01AH
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Quote:
Originally Posted by MadderThanMax
Brake Servo may not be operating as effectively as it should.. this would cause a drop in pedal..If it has a hose connection to it, with the engine off pull the hose out, It should *pop* if working properly if it hisses then its a fail and needs replaced...
I pulled the hose off and it popped followed by a short high pitched hiss. I'm not sure if the failure hiss should be drawn out or short. What do you think about this?
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Unread 12-21-2013, 12:43 PM   #10
MadderThanMax
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If it popped, and its a loud, clear *pop* by the way then I'd say that your ok. Its when you pull the hose and it just hisses, then worry..

The *pop* is created by a perfect vacuum in the servo - The Hiss indicates a pin hole or tear in the diaphragm inside the servo. The latter would happen immediate the hose was removed.. so I'd say that your good to go and maybe looking at rebleeding the brakes...
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Unread 12-21-2013, 08:46 PM   #11
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So after eliminating the booster today, I can only believe that there is air in the lines still. I will be going by the shop in Monday to ask them about their bleeding procedures. So far my questions are 1) did the bench bleed the master before they installed it 2) what was the wheel position order they bled them 3) did they bleed the abs system with an obd2 tool? Can anyone think of any other questions I should ask? I'm also looking into getting that hard line replaced. It looks terribly brittle.
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Unread 12-22-2013, 03:08 AM   #12
MadderThanMax
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That last question is a bit ambiguous "An OBD2 Tool" you're liable to get that look of horns on your head... An OBD2 tool only reads/resets the codes nothing else...
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Unread 12-22-2013, 03:10 AM   #13
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What happens when you pump the pedal?? does it get firmer?
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Unread 12-22-2013, 04:37 AM   #14
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The bleed tool is an OBD3, which cost thousands and they probably do not have.
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Unread 12-22-2013, 10:08 AM   #15
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Thanks for the correction. I will probably have to go to the dealer for the OBD3 then. Also, when I pump the pedal with the car on, it does not get any firmer. When I pump the pedal with the car off, it hardens up like it should.
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