A few days ago, I noticed that when I first step on the brakes on my '84 300SD (no ABS) they feel firm, but then as I wait further at the traffic light for which I braked, they start to soften and loosen and I'd then have to push the pedal almost to the floor to completely stop the car. Sometimes this would happen, some other times it wouldn't. The situation is getting worse as sometimes even as I approach the light, the brakes would be soft right away and I'd have to push them to the floor to stop the car.

My mechanic and I closely inspected the situation. For some strange reason, the level of the brake fluid in the reservoir had risen!! Just less than two months ago, I had the rear brake pads and rotors replaced. The front ones, while thin, are still usable, so that's not the cause of the problem. My mechanic drained the excess fluid from the reservoir. Upon pumping the brakes with the car turn off, we could hear a noise (sounded like a leak) that comes off the master cylinder when the brakes are pumped. At that point he concluded that I needed a new master cylinder. He explained that the failure of the pistons in the master cylinder to hold pressure was resulting in the fluid not recirculating as it should, thus the increase in the amount of the fluid in the reservoir.

I did several searches here, and while there was no thread that directly addressed my issues, I learned that (1) it can't be the brake booster as the pedal would be really hard if that were the case (2) considering that my car had just 145K when I purchased it, 20 years after its manufacture, such premature wear of the master cylinder isn't unusual as the car was mostly driven in the city all its life. Are all these correct?

I intend to keep this car as long as God allows me to. So I should just buy a brand new, not rebuilt one, right?

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1999 Mercedes-Benz S600, 103K miles - garage queen
1988 Mercedes-Benz 560SEL, 89K miles - daily driver
2007 Hyundai Sonata Limited, 31K - daily driver