Functioning brakes stop a vehicle by using friction. In this way, they are unlike the engine, which must always be kept lubricated to run smoothly. In front brakes, friction stops the brake calipers and pads. In rear brakes, friction hits the brake drums and shoes.

Several factors can interfere with this friction and lead to brake failure:

• Grease or oil on brakes causes brake failure, because it interferes with friction. If oil leaks, it may indicate that an oil seal has failed.
• When the brakes overheat to a great degree, the metal in the brake rotors or drums develops hard spots. These are known as hot spots. The hot spots resist the friction from the brake shoes and pads. Because the shoes or pads have nothing they can grasp, there’s no friction. Consequently, braking power is lost.
• Brakes that squeal indicate that the brake pads are wearing thin. By the time the brakes begin making a grinding sound, they’ve worn down past the pads to the rotors, which will cost more than pads to replace.