Losing Brake Fluid Slowly
Daughter drives a 2006 with 52K on the clock. About nine months ago (just after the semester started) I noticed that her brake fluid level was way down.
Each time she came home (or I went there) I checked it and topped it off.
There were no visible signs of leakage either at the Master Cylinder itself or at the wheels. I had a mechanic that I trust in her college town look at it, and he couldn't find any evidence of a leak. In December 2010 I took it to the best dealership in our metro area, and they could not find a leak, but wanted to replace the master cylinder for $300. They were less than forthcoming on their explanation, and wouldn't express a strong opinion that the replacement would solve the issue, so I passed.
The spring semester went much like the fall. I had boyfriends, cousins, and whoever happened to be at the college town checking the fluid and topping it off until May.
This last weekend I purchased a new master cylinder (not reman) for $135,
and attacked the project Saturday morning. I started at 7am due to our heat wave, in hopes of finishing before the real heat arrived.
Removing the block of fuses that was in the way was the most frustrating part of the exercise. The brake lines came loose easily, ditto the two MC bolts holding it to the booster. The instructions with the MC told me to bench bleed it, and I tried and tried to get all the bubbles out (unsuccessfully) so I resigned myself to bleeding the brakes.
When the old MC came out it was apparent that the fluid had been leaking out the piston seal and running down into the booster, and (I guess) burned off through the engine. I buttoned it back up, and with the help of a friend, bled off all the old, discolored fluid through the wheel cylinders one by one.
This actually turned out to be a good thing, as now the entire system has new, clean fluid throughout. The bleeding was not as complicated as I recall the project being, so now we'll wait to see if the mysterious slow leak is resolved.
I have owned multiple vehicles (Toyota Honda Jeep) that have been far older than the Cobalt and have never had a MC go bad.