The reason for replacement is what Matt said. The purpose of the drier is to remove moisture from the refrigerant and it can only do it as long as it has not absorbed tons of moisture already which is what would happen when the a/c system is opened and kept open for an extended period of time and also during flushing. If you dont replace the drier, excessive moisture in the system would damage the compressor and also fine tubing in the condensor as well as the evaporator. Replacing the expansion valve is also a good idea though not absolutely necessary. And ofcourse replacing all the seals (o-rings) around tubings that are opened should always be done. Replace the inserts of the schrader valves also.

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Saumil S. Patel