When attempting to push button start our 2017 1.5 LT; I've noticed it is very difficult to depress the brake pedal while attempting to start the engine. It seems as though there is no residual vacuum in the brake booster to allow the pedal to be depressed easily. I'm wondering if others operate this way, or that I may have a leaking booster or faulty check valve allowing the vacuum to escape after turning off the engine. Typically the brake pedal can be depressed a couple of times with the engine off, and still have assist from the brake booster. My pedal doesn't depress at all without the engine running.
GarBec
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My Gen7 will retain its vacuum overnight and even over a weekend. It has 113,000 miles and counting. You might want to speak with your service department about it.
There are 2 general types of brake assist: vacuum and hydraulic. I have seen some of the more expensive cars use hydraulic pressure from the power steering pump, but they have to use a small high-pressure tank to accumulate pressure for use if/when the engine is off. Those systems routinely get about 1 brake application without the engine spinning and then they become almost unmanageable. All of the vacuum systems I have ever seen have always used a large booster housing to store "vacuum" for use whether the brake is being applied while the engine has no manifold vacuum, such as during WOT (wide-open throttle) or after the engine has been turned off. With those I have always gotten 2 or 3 brake applications, and sometimes more.
If you park your car and remove your foot from the brake and then turn the car off, there should be vacuum left in the booster the next time you step on the brake, whether it's the next day, the next week, or even the next month.
If, however, you park your car and leave your foot on the brake, that action could be what is consuming the stored boost that should be available. Observe how you park your car and then see if changing how you do it leads to a difference the next time you start your car.
If you still have no boost left, or if it's still so minimal that it feels like there's nothing there, then get it into the dealer. There's a possibility that you have something wrong that could fail while you're driving it.
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The term "vacuum" is actually a misnomer. It is actually a pressure difference. Outside the engine we live in an atmosphere with the pressure of (wait for it) ONE ATMOSPHERE, or 1 atm. When your engine is running and the throttle plate is not fully open the pressure inside the engine will be less than 1 atm.
The measurement we tend to see on our "vacuum" gauges is Inches of Hg with a second scale of mBar. — (Image) It is a measurement of the pressure below 1 atm and we, being humans, call it "vacuum".
Notice that the only place on the gauge that suggests that the vacuum is actually registering as a negative pressure is the minus sign before the "30". Also notice that the hand on the gauge rotates counter-clockwise, since that is in the direction of lower pressure. Higher pressure (aka, "less vacuum") makes the hand rotate clockwise. ("Clockwise" is assumed to mean "to the right" since this was written in North America where our clocks turn in the right-hand direction, as opposed to Germany where they turn in the left-hand direction.
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If you park your car and remove your foot from the brake and then turn the car off, there should be vacuum left in the booster the next time you step on the brake, whether it's the next day, the next week, or even the next month.
If, however, you park your car and leave your foot on the brake, that action could be what is consuming the stored boost that should be available. Observe how you park your car and then see if changing how you do it leads to a difference the next time you start your car.
If you still have no boost left, or if it's still so minimal that it feels like there's nothing there, then get it into the dealer. There's a possibility that you have something wrong that could fail while you're driving it.
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The term "vacuum" is actually a misnomer. It is actually a pressure difference. Outside the engine we live in an atmosphere with the pressure of (wait for it) ONE ATMOSPHERE, or 1 atm. When your engine is running and the throttle plate is not fully open the pressure inside the engine will be less than 1 atm.
Click here for website quoted above
The measurement we tend to see on our "vacuum" gauges is Inches of Hg with a second scale of mBar. — (Image) It is a measurement of the pressure below 1 atm and we, being humans, call it "vacuum".
Notice that the only place on the gauge that suggests that the vacuum is actually registering as a negative pressure is the minus sign before the "30". Also notice that the hand on the gauge rotates counter-clockwise, since that is in the direction of lower pressure. Higher pressure (aka, "less vacuum") makes the hand rotate clockwise. ("Clockwise" is assumed to mean "to the right" since this was written in North America where our clocks turn in the right-hand direction, as opposed to Germany where they turn in the left-hand direction.
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You're 100% correct. If you press the pedal and keep your foot still it won't consume more of the vacuum to help you move the pedal.
There are other valves in the system aside from the one-way valve where the big vacuum hose attaches to the booster housing. One is between the brake pedal and the booster's vacuum compartment. Another is between the two sides of the booster. When the pedal is pressed, the last 2 valves are actuated. One seals off the 2 sides of the booster and the other allows atmospheric pressure to enter the driver's side. By increasing the pressure on the driver's side, the pressure differential helps apply the brakes. When the brake is released, the atmospheric valve closes and the 2 compartments are connected, which is what consumes ½ of the remaining vacuum.
I found some pictures on the internet that are not mine, but since they're out there I'm using them to help you guys understand how they work.
In the first one the green lines show where the air enters the compartment. The second is probably a professional drawing and just shows more mechanical details.
Click for video #1
Click for video #2
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Let us know how it finally turns out and whether you get any pushback from your service professionals. You shouldn't, but sometimes humans will be humans.
You can tell from the participation that those who contribute thoughts or ask clarifying questions really have only one goal in mind: to arrive at a successful resolution to the issue presented. I really think that this is what prompted me to buy a used Gen7 after I lost my beautiful '11 LTZ to a flood. It's just odd that it was another '11 LTZ, with the same 3.6L V6 and the same color! (Not complaining!!!)
Glad it helped you understand your car better and hope it helped when you were there in the service department.
Let us know how it finally turns out. I'll bet starting the car will be more of a dream than a nightmare when it's done!
Let us know how it finally turns out. I'll bet starting the car will be more of a dream than a nightmare when it's done!
With that new booster assembly, did it come with a new 1-way check valve and a new grommet? If not, that could be the whole issue.
Or not...
It sure sounds like there's just one little piece of this puzzle that's being overlooked, and it's probably been staring us all in the face the whole time!
Or not...
It sure sounds like there's just one little piece of this puzzle that's being overlooked, and it's probably been staring us all in the face the whole time!
No matter what they say, I'd press for more than just the rhetoric, too. No brake system from any manufacturer, including GM, has ever acted like this. It's defective somehow. They need to dig into it and figure it out, then make it work properly. I encourage you to keep on it and them. You'll do good for yourself as well as others that come after you.
Your theory sounds very plausible!
It wouldn't surprise me if they found out that someone welded the attaching pin on the brake pedal arm just a leeeetle too close, or used a connecting rod that's a leeeetle too long, causing the whole shebang.
It wouldn't surprise me if they found out that someone welded the attaching pin on the brake pedal arm just a leeeetle too close, or used a connecting rod that's a leeeetle too long, causing the whole shebang.
You can also find some info in the following threads:
https://www.chevymalibuforum.com/forums/121-problems-service-issues-troubleshooting-generation-9/110433-brake-pedal-squeak.html
https://www.chevymalibuforum.com/forums/121-problems-service-issues-troubleshooting-generation-9/108298-brake-pedal-squeak-press-release.html
About 2 years ago I contacted them, first by PM, then by email, and then via phone. I related how they seemed by many, including myself, to be ineffective. Their responses were canned and showed no real understanding of the actual issue presented, no evidence of any resolution seemed to be shared, and very few ever reported hearing back from them. I told them that their value and/or effectiveness was determined by what we could see, which was an inconsistent presence and boiler-plate "answers". I felt like what I received was more of the same, since nothing really changed.
Their last post was May 2, 2017 at 16:30... Feeling forgotten now?
"One size doesn't fit all."
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