It goes without saying that your brakes are likely the most important system on your car. Without them, pretty much everything else you do to your car is pointless seeing as how you could lose it all if they fail. Luckily, total failure isn't very common. So what are the more common ways that brakes can fail?
1. Seized calipers.
Brake fluid is hygroscopic, meaning it absorbs moisture from the air. Over time, in an under-maintained brake system, water builds up in the brake fluid, causing corrosion of steel components, namely, caliper pistons and the bores they move within. On Benzes from the 50s through the 90s, virtually all came with two-piston calipers that squeeze the pads together on either side of the brake disc. Commonly, one will seize (although both can) and cause one of the pads to rub constantly on the disc. This will, at best, lead to premature wear of the pad. However, it can also cause pulling to one side on braking, reduced fuel economy, and sometimes even full brake system failure. This occurs because the constant friction of an applied brake pad will heat up the caliper which can then cause the brake fluid to boil. Once it boils, your pedal will go to the floor as if there were no fluid in the system whatsoever. All you can do is hope it doesn't happen when you really need to stop. If this happens, you can downshift to use the engine compression as a brake, and you can also use your emergency brake. It helps to run through this scenario occasionally in your mind so that if the time ever comes, you react automatically.
Calipers can also lock up due to old brake hoses. As they age, they swell internally. Eventually, they will essentially close up. The problem is, they can be forced open by hydraulic pressure from the master cylinder, but it becomes a one-way avenue. Pressure makes it through the hose to force the caliper to squeeze, but then the occlusion of the brake hose traps some of that pressure in the caliper causing it to constantly brake, at least somewhat. Then you're back to the same scenarios as listed above.
Brake hoses come with their manufacture date either printed on them or even formed into the rubber. Either a month/year or week/year format. I find that hoses are often still just fine at even 10 years old, but if you're getting above that, it's time to make a plan to replace them. Luckily they're not usually very expensive. The date is not always visible, so you can also look for cracking and stiffness as signs of age.
2. Leaks
If you have a soft brake pedal and the fluid in your reservoir is low, chances are you have a leak. Leaks can occur various ways. Commonly, you'll see wetness on the caliper and/or the associated wheel meaning that the caliper itself is no longer sealing in the fluid. Probably the second most common leak point on an older car is one of the steel lines. For the front brakes of a non-ABS Mercedes, you have a dedicated steel line for each of the front wheels coming directly from the master cylinder. For the rear brakes, you have a single steel line that goes toward the rear of the car parallel to the steel fuel lines (it's the smaller line of the three). It then splits off to each rear wheel. This means that a leak in any point of the rear brake lines will cause you to lose rear brakes completely.
Leaks occur more commonly in the rear lines because they are more exposed to the elements. They are made of a coated steel which makes them much more durable than the lines in, say, your Chevy truck, but all it takes is a rock chip or scratch in the coating and water can come into direct contact with the steel, kicking off the slow but sure process of corrosion. Eventually a hole forms and that means you need a new brake line, or at least a new section of it. ABS cars have a dedicated line for each wheel but basically all of the above applies.
What to do
If you suspect that you have a seized caliper, one quick test is to walk around the car after at least a few miles of driving and put your hand to each wheel. If one is noticeably warmer than the others, you have probably found the culprit. From there, it's a matter of replacing the caliper AND its counterpart on the other side with new or rebuilt calipers. Some people will want to just replace the seized one but this is not recommended.
The better option is to prevent it. This is done with regular changing of the brake fluid, as in every 2-3 years. New brake fluid is about the color of extra-virgin olive oil. If yours is dark, it's time for a change. Often I find that people acquire a car and have no record of it being done. In this case, unless the fluid is obviously new, it's best to have it done and then start keeping track from there. A maintenance spreadsheet is easy and very useful.