Each brake pad is constructed of a metal shoe (backing plate) on which the lining is riveted. Brake pads wear out during the time with the use. If the linings wear down to the metal shoe, then there will be a metal-to-metal contact where the shoes rub directly against the disc causing severe damage and loss of braking efficiency. So the brake pads should be checked for wear periodically and replaced if needed. There are many qualities and types of brake pads available in the market. The differences have to do with brake life and noise. Harder linings tend to last longer and stop better but they may produce irritating squeal when applied.