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2006 LT 2.2 sat for a month started it up and brake pedal goes to the floor, pumping doesn’t help, moved the car a couple times and the brakes came back. Prior to this problem the brake light came on a few times so fluid was added although there isn’t a leak. A couple of 12oz bottles have been added over the past few months! I have no history on this car, I have owned it a couple weeks now.
 

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had a master cylinder rear seal leak for awhile, about 100mls every few months, once i verified it wasnt leaking anywhere else i came to the conclusion it was the master cylinder.
brakes worked fine but had to keep an eye on brake fluid level. pia to get so i procrastinated on removing the master cylinder as the fuse box was in the way and had to be removed before i could tackle it.

next issue that developed was a check engine code mil of p0171 retrieved using a scantool, a lean condition code as fuel trims maxed out on idle and somewhat got better under throttle.
i did the water and propane air intake vacuum leak test and found, after some considerable time, that my brake booster had cracked which was causing the vacuum leak/air intake leak hence the p0171 and progressively worse rough running car.
so time to finally tackle the job. 1st video is the fuse box removal, 2nd video is the steps in removing the brake booster/master cylinder



 

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Discussion Starter #3
Damn bro, thank you that’s sounds like my exact scenario down to the running roughly.
 

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I have replaced the master cylinder on the Cobalt and Malibu with the issue where the brake fluid will enter the brake booster through the rear side. You only find out once you’ve removed the master cylinder. Also brake cylinders can do this. They will not always leak out of the drum and onto the wheels. Sometimes you’ll need to remove the rear drum and inspect the inside. I do plenty of brake repairs so I see this often when a car has sat for a little while.

For instance, I left my Lincoln Town Car parked and only just start it without driving it. It’s brake fluid has dropped. Although I haven’t found a for sure cause, I believe the brake lines may be leaking as they are old and rusty. Although there’s no fluid showing on the frame or around the wheels. But I already had that on the to dos after it gets repainted.
 
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