Originally Posted by tbomachines

Well, if there are any leaks that would be the cause. And, you will find your fluid level slowly going down. If there is not a leak in the lines somewhere, then your master cylinder is most likely bad. I had this happen in one of my 300SDs, fluid was leaking into the brake booster. IF there is fluid at the bottom of the brake booster, replace both the MC and the booster, as brake fluid is extremely corrosive and will eventually cause the BB to rust. Used BB is the way to go in that case, as new they are very expensive! Replacing the MC - hour or 2, BB - an hour at most. Bleeding and making sure there are no leaks - a few hours, be meticulous!