Figure 7-14. - Self-energizing and servo action.

problem that occurs during heavy braking is brake fade. During panic stops or repeated harsh stops, the brake linings and drum develop large amounts of heat that reduces the amount of friction between the brake shoe and drum. This reduction in friction greatly decreases the stopping ability of the vehicle, and, in most cases, additional pressure directed on the brake pedal would not increase the stopping performance of the vehicle.

The enclosed design of the brake drum assembly does not allow for cooling air to enter the assembly and therefore heat developed during braking must be dissipated through the brake drum and backing plate. As the brakes heat up due to repeated application, cooling air flowing past the drums and backing plates is limited. This condition causes the radius of the drum to increase more than the radius of the brake shoe. As a result, a change in pressure distribution between the linings and the drum occurs, which reduces the braking ability of a vehicle by up to 20 percent.

The enclosed design also does not allow for water to be expelled rapidly should the brake cavity become wet due to adverse weather conditions. The water reduces the frictional properties of the brake system and must be removed to restore braking ability. This is a very dangerous situation and drastically reduces the stopping ability of the vehicle until the system is dry.