We don't think it's entirely outrageous to call the BMW brake caliper the most important part on your vehicle. After all, that fast, sharp handling Teutonic driving machine isn't squat if you plow it right into the side of a building because you can't stop. And the BMW brake caliper is, for the most part, the thing that makes you stop. Sure, the BMW brake caliper is only one component (or, more accurately, four components), in the braking system, all of which have to work together to bring your car to a safe stop. But as the chunk of iron that actually grabs hold of the brake rotor, the BMW brake caliper gets our honor as the workhorse of the whole system. For the most part, each BMW brake caliper is fairly trouble-free, going about its business for tens of thousands of miles between overhauls. But the BMW brake caliper has an Achilles' heel: It's full of fluid, and eventually that fluid finds its way out of your BMW brake caliper. The rubber seals deteriorate over time, or the brake fluid doesn't get changed often enough, leading to internal rust in the BMW brake caliper. Whatever the cause, a leaking BMW brake caliper doesn't hold pressure adequately and tends to foul the brake pads by slinging fluid all over them. In the end, you'll need to replace the BMW brake caliper with a remanufactured unit or a new unit. When you do so, you'll probably want to replace it as part of a BMW brake caliper set, including two calipers and brake pads. That will ensure that you get straight, smooth, trouble-free braking performance for tens of thousands of miles more.