All hail master cylinder! Your BMW Master Cylinder should be way up there on the list of things you don’t want failing at an inopportune time. After all, the master cylinder is at the top of the food chain as far as your brakes are concerned. When you press on the brake pedal, you’re pressing on a piston in your BMW master cylinder (via the power booster in most cases). The BMW master cylinder, in turn, sends fluid to the brake calipers, which clamp down on the rotors, bringing your car to a stop. Pretty important stuff.

If your car was made after 1967, your master cylinder is a dual-circuit design, which provides a nice fail-safe mechanism for your brakes. The BMW master cylinder is actually two master cylinders in one. That way, if you get a severe leak in your rear brake lines, the front brakes will still work, and vice versa. If you get a severe leak in both front and rear brake lines at the same time—well, even BMW engineers didn’t plan for your ex-wife. That’s what the emergency brake is for.