- 2007 LS
- 2.2 liter EcoTec engine
- auto trans
- a little over ~101k miles

So I have a peculiar "intermittent" issue with the braking power - seems like the braking power "varies" towards the end of the day, instead of being constant. I notice it's worse on the hot days for whatever reason...

When the car is cold (say... in the morning), the brakes seem to work just fine when I'm shifting out of "P" into "R" - the car stays put/stays still with my foot on the brake (working as intended)

But after a long day of driving around (running errands, commuting to work, stop-and-go traffic, etc) and the car just warming up in general... after the car has been turned off for a bit and I go inside for an hour or two, then I come out again for a quick fast food run (for example): I get in the car and it starts right up as usual, but when I put my foot on the brake pedal and shift from "P" to "R" I notice the car starts moving even with my foot pushing down on the brake! 😳 The car moves a good 5-8 inches before I feel the brakes start to "bite" or "kick in". And while this is happening I notice the brake pedal feels soft/spongey. In said parking lot scenario I also notice the brakes don't seem to do anything as I shift to "D"

Then, as I'm driving down the nearby side-streets (for example), I notice the braking power "slowly return to normal" as I'm pushing down on the brake pedal here and there. Usually by the time I get to the intersection stop light a few blocks away the brakes have mysteriously "returned to full power" and the brake pedal once again keeps the car put/in place

As for the physical status of the brakes themselves:

- the front brakes/pads are brand new as of June 2023 (installed by a reputable shop, so I have no reason to believe they were installed incorrectly)
- the rear drums and shoes are "at about 70% life left" according to the shop that cleaned/adjusted them earlier this month (September)
- brake fluid flushed/refilled in December 2022 (a different shop did this)

As for other observations:

- I kneeled down and took a good long look at the pavement under the car with a flashlight, but did not see any obvious leaks on the ground, pavement was dry
- no current codes present according to my scanner besides a generic "B0081" seatbelt code, the MIL light is not turned on
​​
What's going on here boys? Any ideas? Vacuum leak maybe? And how would you test something like a brake booster anyways?

Been dealing with this intermittent issue for a few weeks now, not gonna lie it does feel a bit unsettling when you step on the brake pedal in a parking lot but the car moves/creeps forward anyways 😲

 

Could be swelling brake flex hoses, a bad power brake booster, bad brake fluid , worn brake pads, sticking brake caliper, sticking caliper slide pins. When was the last time you adjusted the rear brake shoes? How many clicks to set the parking brake, should be no more than 3

 

I thought I already mentioned in the post, rear brakes were adjusted/cleaned earlier this month (September)

 

You mentioned a shop reported they were clean and adjusted, the important thing is the EBrake lever, how many clicks to set the parking brake?

 

Well, the mechanic said my rear brakes were "at about 70% life left", he didn't mention anything about the parking brake. It was a fairly short visit, I think I was in and out in about an hour (around 45 minutes I think?). Because I'm a regular, he gave me a discount and only charged me 1/2 hour

Is the thing about clicks something I can check myself? Or will I have to go back to the shop?

 

Just count the clicks as you apply the parking brake, vehicle stopped transmission in park. Sounds like they looked at it but didn’t adjust it , at 70% life left, they are probably the OEM brakes back there, I bought a full rear brake kit drums, shoes and all hardware, a great improvement in braking, the inspection work order said the rear brakes were 75% life left in them, but they were 9 years old!