Most freon leaks occur in the evaporater coil inside the air handler. This is due to rust on the header plate of the evaporator coil. The rust punctures the copper lines on the coil. Leaks can happen about anywhere else in the system due to vibrations and rubbing. There are other parts that do rust out and cause leaks including dryers, accumulators and compressors. These components do not develop leaks as frequently as the evaporator coil. Very rarely do leaks occur in the line set itself in between the condenser and air handler.