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Brake pedal goes to the floor. When it's not running the pedal feels fine

Tiny
EROSE0120
  • MEMBER
  • 2005 CHEVROLET IMPALA
  • 3.4L
  • V6
  • 2WD
  • AUTOMATIC
  • 216,000 MILES
I changed my front rotors and cleaned the back brake pads. Four wheel disc. Afterwards when running my brake pedal goes to the floor. When it's not running the pedal feels fine. Then I changed the master cylinder and bled m c and lines with no improvement. The car kind of stops but not too well. Any help or ideas would be appreciated.
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Wednesday, May 15th, 2019 AT 6:12 PM

9 Replies

Tiny
SCGRANTURISMO
  • MECHANIC
  • 4,897 POSTS
Hello,

This could be a problem with a split in one of your vehicle's brakes lines. Your brake system is essentially a hydraulic system that needs to be sealed and free of air to work properly. So if you have bled your brake system properly of air pockets, then there is something that is bleeding off the pressure somewhere. I have included a link for you to go to below:

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

Please double check the lines for leaks and get back to us with what you are able to find out.

Thanks,
Alex
2CarPros
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Wednesday, May 15th, 2019 AT 7:35 PM
Tiny
EROSE0120
  • MEMBER
  • 4 POSTS
The link you sent did not work. There is no leak whatsoever. I've inspected the lines from each caliper all the way to the master cylinder. As I stated before when the engine is off the pedal is firm with no movement at all. Then when I start engine the pedal goes to the floor.
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Wednesday, May 15th, 2019 AT 11:48 PM
Tiny
SCGRANTURISMO
  • MECHANIC
  • 4,897 POSTS
Hello again,

Sorry about that. Here is the link again:

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

Please go through the guide and then get back to us.

Thanks,
Alex
2CarPros
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Thursday, May 16th, 2019 AT 2:39 AM
Tiny
EROSE0120
  • MEMBER
  • 4 POSTS
I've checked all these things so far. I have no leaks. I've changed master cylinder. Bled entire system. Have four wheel disc brakes so no drums in the back. No matter how many times I pump the pedal it doesn't get firm. It only gets firm when car is shut off.
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Thursday, May 16th, 2019 AT 3:09 AM
Tiny
SCGRANTURISMO
  • MECHANIC
  • 4,897 POSTS
Hello again,

Have you check the vacuum hose for a leak? Okay, anyways I have included in the diagrams down below a factory Brake Assist Troubleshooting Guide for you. Please go through the guide and get back to us with what you are able to find out.

Thanks,
Alex
2CarPros
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Thursday, May 16th, 2019 AT 12:32 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 47,602 POSTS
If I can jump in here do you have ABS? If so it sounds like you need a scan tool to open the ABS valve while bleeding.
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Thursday, May 16th, 2019 AT 3:10 PM
Tiny
EROSE0120
  • MEMBER
  • 4 POSTS
I do not have ABS. Vacuum works good but booster doesn't hold pressure. The check valve and gasket seem to work. When I sucks on vacuum hose to booster it seems to hold and I can't blow through the same hose.
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Thursday, May 16th, 2019 AT 4:46 PM
Tiny
SCGRANTURISMO
  • MECHANIC
  • 4,897 POSTS
Hello again,

Sorry for not sending the troubleshooting guides. I must of had a glitch on my end with something. Here they are in the diagrams down below:
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Thursday, May 16th, 2019 AT 9:26 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 47,602 POSTS
The valve can be bad inside the booster. also when you replaced the master did you bench bleed it? if not here is a guide to help:

https://www.2carpros.com/articles/how-to-replace-a-brake-master-cylinder

You must start with a properly bled master.
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Friday, May 17th, 2019 AT 3:29 PM

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