WHAT DOES A DRIER DO- DRY?
IT'S NOT THE FREON!! It's the oil in the refrigerant that absorbs moisture and holds debris in the system. Replacing the drier / accumulator, in addition to evacuation, will assure better performance.

Why Replace the Receiver-Drier?
The receiver-drier must be changed each time a system is empty regardless of the reason for loss of refrigerant. It should also be changed every three years, because the desiccant pellets will break down and clog the expansion valve. This will in turn cause the system to become inoperable and May damage the compressor.

The receiver-drier is strictly a disposable item and is thought of in the same terms as a fuel, oil, or air filter. In fact, if any component fails or is replaced for any reason, the receiver-drier must also be replaced to prevent corrosion and moisture in the system.

The receiver-drier performs three functions:

-It filters the system of non-condensables.

-It receives the liquid refrigerant and maintains a certain level of liquid at the bottom at all times in a properly charged system.

-It contains a stack of pellets called desiccant (drying agent) to trap and absorb moisture. NOTE that moisture is the most harmful enemy of the air conditioning system. If any moisture is in the system, it will combine with the refrigerant to form hydrochloric acid which is extremely corrosive to metal components.

Replacing the receiver-drier is essential when servicing the A/C system. Whenever you replace a component of the A/C system you must also replace the receiver-drier. If you do not change the receiver-drier there could be serious damage to the other parts of the system, which could be very costly. You must also have proof of changing the receiver-drier in order to receive a compressor warranty.

WHY INSIST ON FLUSHING
There is no way to know how much oil is in a system
(1) Flushing eliminates excess oil from previous services. You do not want to add another full charge of oil when replacing a compressor and adding oil to the system.
(2) Sludge and debris, left in the system, clogs the orifice tube/expansion valve and can damage the valves in a compressor.