The squeezing ability of the BMW Disc Brake Caliper is how your vehicle is brought to a quick and safe stop. The disc brake caliper is on the front wheels of all BMW and on selected models they are also on the rear wheel assemblies.
The BMW disc caliper receives the pressurized hydraulic fluid from the master cylinder and converts it to mechanical motion. This is made possible by the movement of the piston inside of the disc brake caliper. The piston moves outward which is what pushes the brake pads against the rotors which stopped your BMW from moving.
Around the piston in the disc brake caliper is a polymer seal. This seal is in place to keep the hydraulic fluid contained in the caliper and not permitted it to leak out if the seal is function correctly. Because the seal is made out of a polymer, it will dry out over time and cracks will develop in it. Through the cracks is where the hydraulic fluid will leak out causing this braking component to fail.
The driver of the BMW will notice the leak in their braking system when the brake pedal feels soft and the pedal has to travel farther for them to receive the same pressure to stop the vehicle as compared to when the system is operating correctly. With the seal leaking from around the piston, brake fluid can be noticed on the disc and over most of the rest of the wheel assembly.
If the owner of the vehicle has ever preformed a brake job on their vehicle, they can replace the disc brake caliper themselves. This will restore the closed loop system of their braking system so it can function properly again.