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Brake problem - sinking pedal

ErnieX

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Explorer bit the dust. Have 2008 Merc Mariner. More action here than Escape forum. Brakes started sinking to the floor and not holding pressure when stopped. Seemed to be typical master cylinder failure. Replaced MC w/new Ford made MC, bench bled, installed and bled system. Pedal still sinks but stops car at bottom of travel. Sinks when holding at a stop. No ABS codes or light and ABS functions as tested on dirt road. No leaks anywhere in the system. New pads and shoes, hoses appear to be fine. I'm stumped. Tested MC when bench bled and it seemed to hold pressure as it should. Can ABS system cause this? Power booster seems to be okay. Anything else in the brake system that can cause this? Am I missing something?

Thanks for any help,

ErnieX
 
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Flag Gibby

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Does the engine stumble or idle high when on the brakes? Just wondering if the vacuum line to the MC is bad. I know that the ABS lets the system pressure bleed down some once the vehicle is stopped and you keep your foot hard on the pedal but nothing like what you are talking about. I would think that if the ABS pump was not working that you would have a DTC and an ABS dummy light lit up on the dash.
 
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ErnieX

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Thanks for the reply;) No, no change in engine speed or idle when brakes are applied. Also, forgot to mention, brakes don't pump up like they will do when there is air in the system. Occasionally pedal will go all the way down with no pressure until bottom of its travel, but will stop.

Other thoughts?
 
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Flag Gibby

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Wait, you replaced the master cylinder, but not the brake booster? Your original symptoms indicate a bad brake booster and not a bad master cylinder to me. Typically a sinking pedal is due to a brake booster that has a leak and cannot maintain pressure, hence why it leaks down quickly.
 
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ErnieX

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Thanks again, Flag. Ordered a new booster. Be here in a few days. Nobody local stocked it. Will post back with results.
 
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96eb96

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A bad abs accumulator could cause this if the M/C didn't solve it. I heard of rare cases where the brakes can completely fail because of that. If one of the valves are sticking open that is the symptom you could get.

A bad brake booster would give you no assist. You would have a very hard pedal.
 
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ErnieX

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Replaced the brake booster out of desperation for a fix. Bled the crap out of the system again and tested. The brake pedal now goes all the way to the floor before engaging the brakes. Car stops with power assist but have to push it all the way down. Disconnected and plugged the vacuum line to the booster to take it off line. The pedal is hard, brakes function properly but need a lot of pressure as would be expected with manual brakes. How can this even happen? As you said, a bad booster would only result in a hard pedal and pedal doesn't go to the floor with the booster disconnected. ABS function properly and no warning lights for brakes or ABS. Could the ABS accumulator still be the problem when manual braking functions properly?

Any help greatly appreciated.
 
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Flag Gibby

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Did the new brake booster come with a new check valve already installed on it? It's an $8 part and what I should have been originally thinking of and not the whole brake booster like 96eb96 pointed out.

See here for a good description of how the booster works and some troubleshooting tips:

http://www.agcoauto.com/content/news/p2_articleid/129
 
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ErnieX

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Thanks Flag. Excellent website with lots of good explanations. I bookmarked it for future reference. The new booster did not come with a new valve. The valve is an inline valve in the middle of the vacuum line. I did however test it and it functions properly allowing air flow in only one direction. The fact that the pedal is high and hard with the booster disconnected as mentioned above is truly puzzling. Cars going to the dealer next week for a power steering recall so may have to have them check it out if not solved before then.

Thanks again for your input.
 
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Bkeller

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Explorer bit the dust. Have 2008 Merc Mariner. More action here than Escape forum. Brakes started sinking to the floor and not holding pressure when stopped. Seemed to be typical master cylinder failure. Replaced MC w/new Ford made MC, bench bled, installed and bled system. Pedal still sinks but stops car at bottom of travel. Sinks when holding at a stop. No ABS codes or light and ABS functions as tested on dirt road. No leaks anywhere in the system. New pads and shoes, hoses appear to be fine. I'm stumped. Tested MC when bench bled and it seemed to hold pressure as it should. Can ABS system cause this? Power booster seems to be okay. Anything else in the brake system that can cause this? Am I missing something?

Thanks for any help,

ErnieX

Hey ErnieX- Hope you are well. This is the EXACT problem I am having with my 2011 Ford Explorer with 110,000 miles. Did anything ever get figured out on what the cause was? BUT, in my situation, it only happens in stop and go traffic and if I let it sit for the night the brakes work perfectly again the next morning. This has stumped one stand alone shop and a large Ford Service center.
 
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07EddyB

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Check the flexible lines to the calipers. Have someone push the brake while you are watching them and see if they expand. Only other thing besides above that I can think of.
 
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Drewmcg

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2002 XLS 2WD
Having this same issue. Pedal to floor with new Motocraft master cylinder when start car. I don't see how this could be the booster, if the master cylinder is working properly. Must be a leak elsewhere in the system, but so far I cannot find it.
 
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