Over time many car ac systems may not cool as well as they used to. Before you start of summer many car owners make this discovery. The number one cause of an AC system not cooling well is low refrigerant. But when should a car's AC process be completely evacuated and recharged? As a rule, anytime the AC system has been opened for service. May be when a leaky hose, evaporator or any other failed component has been replaced. Remember that if there is any Freon in the machine at all, it needs to be recovered and recycled just by an AC machine. When a component is replaced, the drier (AKA Accumulator) should be replaced also. The drier has desiccant which absorbs moisture in the system. It's effectiveness is diminished after it's exposed to the atmosphere for any period of time. My websites: