the master brake cylinder needs to replaced so I am being told on 2007 Chevrolet Cobalt

took the car to a local repair shop for vibration in the left rear tire. I was told the rim was bent. not sure how this happened since the car has never been in a wreck. the tech told me that this was happening because the master brake cylinder as bad fluid and it was causing the brakes to engage. I did notice that the brake foot pedal would travel less after driving for a period of time.

Asked by for the 2007 Chevrolet Cobalt
Actually, mechainc may not be entirely wrong! Especially since you notice a difference in pedal travel. Possible booster, master cyl. brake hose(s) keeping residual pressure on brake system! Overheated rotors warped causing vibration. Have seen it before 'ON A COBALT'!!! You should be able to tell if after the pedal feels harder, travels less, if car seem not to roll freely! If this the case, easy check for this is remove brake booster vacuum hose to see if brakes free now. Let a mechanic do the test! The one i fixed was the booster/master cyl. combo. Check with dealer for any service bulletins.
if I just have the fluid replaced. would that help out
Can't say for sure but it's not likely! Does it feel as if the brakes are holding without foot on pedal at times?
yes but only after I have been driving for a while. sometimes after the car has not been used for a couple of days also.
You really need to have a mechanic check this out for you. It's to risky for you to try to diagnose!! I just don't want you to wind up with a NO BRAKES situation! I feel like the booster is most likely the trouble, mechanic can confirm. In any event, do not keep driving it like this, a hell of a lot more damage will result!!!
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I have never heard of a master cyl causing a vibration I would take it to get a second opinion
agree with goodguy and run away from that shop