If disc brake pads are worn down, i.e. have less than 1 mm of pad left, they should be replaced immediately. Fortunately, replacing the pads is usually straightforward. After removing the wheel and removing the retaining pin, you can drop the new pads into the disc calliper body. New pads must be worn-in before serious use to achieve maximum braking efficiency. There are different pad materials: sintered metal last the longest but tend to generate more heat and may squeak slightly when damp. Organic disc pads heat up less and are quieter, but wear out sooner.