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  #1  
Old 03-15-2016, 11:24 PM
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Brake Pedal goes to the floor...

So in the past couple months the brake pedal on the WJ has become really soft and would occasionally go all the way to the floor. Today it started doing it all the time and would not pump up. Master cylinder was replaced last June and the fluid was topped off. Pedal was great until November when it went soft. I was able to notice the following things while driving home today

- When coming to a stop in town, the pedal would be relatively firm when the engine was 700 RPM or above.
- When the engine settled into low idle (500 rpm) the pedal would gradually go to the floor and would not hold it in place while in gear.
- I would feel a slight buzz/pulse in the pedal as it went to the floor.
- No codes or lights on the dash
- Master cylinder is full

Any thoughts? I suspect it is the master cylinder but need some additional input.

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  #2  
Old 03-16-2016, 12:11 AM
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Re: Brake Pedal goes to the floor...

A soft pedal can come due to internal leakage, or air in the line(s). Did you already try to bleed the brakes?
Also an internal leak of (one of the) valves in the hydraulic control unit from the ABS could be the problem. Try and remove the ABS fuses and see if that makes any difference. (fuses #4 and #25 in the PDC and fuse #19 on the junction block).
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Old 03-16-2016, 01:20 AM
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Re: Brake Pedal goes to the floor...

probably master cylinder, when you bench bleed a new master cylinder you can't push the plunger in too far it can damage a seal inside. at first its a slow internal leak inside the master cylinder that gradually gets worst. i had same problem on a ford ranger, at first the pedal would gradually go to the floor while stopped but would pump up. just something to look at, hope this helps. my new master cylinder only lasted 6 months
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Old 03-20-2016, 08:30 AM
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Re: Brake Pedal goes to the floor...

Ordering a master cylinder later today. I think I will replace the flex lines while it is apart.
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Old 03-20-2016, 09:22 AM
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Re: Brake Pedal goes to the floor...

Is it the master cylinder itself or the vacuum booster behind it? Sounds like the later to me.
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Old 03-20-2016, 09:24 AM
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Re: Brake Pedal goes to the floor...

Did you troubleshoot first and removed the fuses? The pulsating feeling in the pedal would make me think towards the ABS HMU and one of the valves operating.
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Old 03-21-2016, 10:20 AM
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Re: Brake Pedal goes to the floor...

Quote:
Originally Posted by Frango100 View Post
Did you troubleshoot first and removed the fuses? The pulsating feeling in the pedal would make me think towards the ABS HMU and one of the valves operating.
I need to do that too. Just buried with work since it is Holy Week. I will try that before I order the MC. I have also wondered about a vacuum leak or bad vacuum booster.
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Old 03-21-2016, 11:02 AM
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Re: Brake Pedal goes to the floor...

Vacuum leak, when large enough, would reduce the function of the booster and the pedal becomes hard to push, but will not go to the floor. It would feel the same with the engine off and pushing the brake pedal several times, bleeding the vacuum away. You will feel the pedal becoming harder to push in. While pushing, start the engine, and you will feel the function of the booster, and the pedal will go down somewhat, but also not to the floor.
Pedal going to the floor completely can only come due to air in the lines, or an internal (or external) leak.
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  #9  
Old 04-01-2016, 04:51 PM
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Re: Brake Pedal goes to the floor...

Just did the fuse removal on the ABS system. No difference.
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Old 04-02-2016, 09:52 AM
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Re: Brake Pedal goes to the floor...

If you removed all three fuses from the ABS (fuses #4 and #25 in the PDC and fuse #19 on the junction block) and it still goes to the floor, then or the master cylinder leaks internally, or the HMU.
You probably go for a new master cylinder now?
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Old 04-02-2016, 08:27 PM
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Re: Brake Pedal goes to the floor...

Quote:
Originally Posted by Frango100 View Post
If you removed all three fuses from the ABS (fuses #4 and #25 in the PDC and fuse #19 on the junction block) and it still goes to the floor, then or the master cylinder leaks internally, or the HMU.
You probably go for a new master cylinder now?
I wanted to post more last night but got distracted. Checked for vacuum leads at the vacuum booster, the throttle body, and at the base of the throttle body as well as at major junctions under the hood. All is good with the vacuum hoses.

After disabling the ABS as you advised, we got the same pedal. The guy who is letting me use his WJ while mine is down also agrees - time for a master cylinder.


Did some digging and they are right around $80-90 for a new one or a bit less for a reman. I am going go with a new one and hope to install it next Friday or Saturday.
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Old 04-04-2016, 05:48 PM
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Re: Brake Pedal goes to the floor...

Master cylinder ordered at Napa and will be in tomorrow. Hope to do the swap on Friday or Saturday depending on those who can help with it.
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