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Brake Travel, I feel a bit of a dip shortly before the stop?

Tiny
DMHUGGINS26
  • MEMBER
  • 2008 CHEVROLET IMPALA
  • 5.3L
  • V8
  • 2WD
  • AUTOMATIC
  • 121,690 MILES
When I approach a stop holding down the brake pedal lightly (at less than 45%), I feel a bit of a dip shortly before the stop. I have had the pedal go really soft twice before (after it hit "the dip"). I'm thinking I have an internal leak in the master cylinder. No codes are showing. Thoughts? Thanks.
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Saturday, February 17th, 2024 AT 1:49 PM

22 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

Chances are, the master cylinder isn't a problem, but anything is possible. If you feel a slight dip or drop in the brake pedal, it could be related to the ABS. However, that would have likely set an engine light.

Since you mentioned the master cylinder, try this. With the engine running and the vehicle parked, press on the brake pedal and hold pressure on it. See if it slowly starts lowering to the floor. If it does, then the Master Cylinder is likely failing, and the fluid is bypassing the plunger. If there is no drop, the Master Cylinder is likely fine.

Try this and let me know what you find.

Take care,

joe
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Saturday, February 17th, 2024 AT 7:09 PM
Tiny
DMHUGGINS26
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I will try that. Are there any other thoughts on what it could be?
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Sunday, February 18th, 2024 AT 4:09 PM
Tiny
JACOBANDNICKOLAS
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Hi,

I have felt similar things when the ABS would actuate. Also, it could be related to brake hardware. Are the brake pads/shoes in good condition? Have you checked to make sure none of the slid pins on the caliper need to be cleaned and lubricated?

Let me know.

Joe
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Sunday, February 18th, 2024 AT 6:53 PM
Tiny
DMHUGGINS26
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The brake pads are quite old. I haven't heard them generate any noise, so I haven't really faulted them. The fluid was also over 2 years old both times they did this. I read that fluid should be changed every 2 years, so I did do that recently. If this was an ABS issue, wouldn't the ABS light come on? Thanks.
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Tuesday, February 20th, 2024 AT 8:44 AM
Tiny
JACOBANDNICKOLAS
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Hi,

If the ABS module is signaling a traction issue falsely, no, it wouldn't know there is a problem; it only knows the information provided.

You may want to check the brakes themselves to make sure the brake hardware or slide pins are still good. A slid pin or rusty/corroded hardware can cause something like this to happen if a pad is sticking.

Let me know.

Joe
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Tuesday, February 20th, 2024 AT 7:28 PM
Tiny
DMHUGGINS26
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One more bit of information I forgot to put in: The excessive brake travel stops after I restart the car.
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Wednesday, February 21st, 2024 AT 6:18 PM
Tiny
JACOBANDNICKOLAS
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That is an interesting addition. Brakes can be pumped to regain a solid pedal. Could be that what you are experiencing?

Joe
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Wednesday, February 21st, 2024 AT 7:01 PM
Tiny
DMHUGGINS26
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Well, that's just the thing. I have pumped them like crazy when the car showed the issue and was stationary but still on, and the pressure never came back. After I restarted the car, pressure returned.
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Thursday, February 22nd, 2024 AT 11:19 AM
Tiny
JACOBANDNICKOLAS
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There is a brake booster pressure sensor mounted on the vacuum brake booster under the hood. (See pic below).

Do me a favor. Check the electrical connector on that sensor. Remove it and inspect the connector pins to make sure there is no damage or corrosion that may be causing an issue.

Let me know.

Joe

See pic below.
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Thursday, February 22nd, 2024 AT 8:14 PM
Tiny
DMHUGGINS26
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Sorry for late reply. I will definitely look into this. I looked up a video of a guy on YouTube who changed out this part and he said he was experiencing the same vehicular symptoms as me. Wouldn't a bad brake booster sensor cause a stiffer brake pedal though?
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Sunday, February 25th, 2024 AT 11:00 AM
Tiny
JACOBANDNICKOLAS
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Hi,

The sensor can cause a stiff pedal or one that is too soft.

Let me know if I can help in any way.

Take care of yourself.

Joe
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Sunday, February 25th, 2024 AT 6:41 PM
Tiny
DMHUGGINS26
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Okay, so I think I found what could be this issue. I took the sensor out, but it looked fine. I will have pictures below. No corrosion that I could see on/in any of the sensor or the wires. However when I removed it, it seemed to leak booster pressure a bit too easy. Just by rotating the sensor to loosen it, I could hear pressure releasing. When I put it back in, the bottom right of the hole was loose. I took a picture of the hole and I can't tell if it is a washer seal issue or an issue with the threads in the hole of the booster. I will attach a picture of the hole and you can zoom in on that bottom right to see what I mean.
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Monday, February 26th, 2024 AT 8:49 AM
Tiny
DMHUGGINS26
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Here is a larger image of the hole.
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Monday, February 26th, 2024 AT 8:55 AM
Tiny
JACOBANDNICKOLAS
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Hi,

Where the sensor mounts doesn't look out of the ordinary. Also, when you turn the sensor, it releases pressure, and you will hear the vacuum. It could be the sensor itself.

Let me know

Joe
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Monday, February 26th, 2024 AT 7:09 PM
Tiny
DMHUGGINS26
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Okay. Well, I suppose I am going to replace that sensor. I saw they typically come with a new rubber grommet too. How sure do you think the sensor is the issue? Just double checking because it looks a bit pricey. Lol Because the problem has been so intermittent, it also will take me a while to tell if that fixed the issue.
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Tuesday, February 27th, 2024 AT 10:37 AM
Tiny
JACOBANDNICKOLAS
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Based on the symptoms, I would think it's a 50 - 50 chance. You may want to scan the can-bus to see if there are any codes stored in the ABS module.

CAN stands for controller area network. Basically, the different modules are tied together via a few wires. This type of scan will retrieve codes regardless of the module storing them.

Here is a link showing how it's done:

https://www.2carpros.com/articles/can-scan-controller-area-network-easy

Let me know.

Joe

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Tuesday, February 27th, 2024 AT 6:12 PM
Tiny
DMHUGGINS26
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Okay. Thank you so much. I really appreciate all the help you have given. Sorry for so many questions. I think I am going to order the part regardless to get a new grommet as well. The sensor was so loose in the grommet that was installed. I think I am going to look into getting a scan tool because of the engine surge issue I have going on as well.
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Wednesday, February 28th, 2024 AT 9:31 AM
Tiny
JACOBANDNICKOLAS
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Hi,

You are very welcome. I hope this takes care of the issue. Also, if there is a leak from the sensor (you mentioned it wasn't tight in the grommet) that could be causing air/fuel mixture issues and the engine surge.

As far as scan tools are concerned, in this day and age, I feel everyone should have one. When you decide to get one, try to find one that will scan the "CAN-bus" system. You can get basic ones for excellent prices. CAN stands for controller area network. A basic scanner will likely only read engine codes. If you get one that scans the CAN, you can retrieve codes from different modules, including the ABS module.

Let me know how things turn out for you or if there is anything I can do to help.

Take care,

Joe
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Wednesday, February 28th, 2024 AT 6:36 PM
Tiny
DMHUGGINS26
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Sorry to bother again. But I need help finding the part. I used a GM parts website, but it almost looked as though this sensor was not listed. Many impalas of the late 2000s I believe have a "vacuum check valve" instead of this sensor. I could be wrong on some of this, but this was what I have inferred or concluded from my research. I then looked up what I thought was the part number, (18085957) but I'm guessing that is just what GM put on to identify what item on the stock impala that was. I can only find like 3 or 4 sensors with that number online and they are all used. The other number I looked up on the part, (the 261230121) I am guessing is the part number used by Bosch to identify the unit. I have found many online with that number that were often made after my car's model year. Is this 261 number the number to go by? Would it be more beneficial to just install the vacuum check valve instead? Thanks.
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Thursday, February 29th, 2024 AT 11:03 AM
Tiny
JACOBANDNICKOLAS
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Hi,

Wow, I see what you mean. I checked as well, and everything indicates out of stock or discontinued. The manufacturer is required to manufacture the part for 10 years after production. So, that means GM is no longer responsible. However, that doesn't help.

Try this. Contact the local Chevrolet dealership and ask if they can locate any dealer old stock (DOS) sensors.

Let me know.

Joe
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Thursday, February 29th, 2024 AT 8:32 PM

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