Remove the air conditioner's condensing unit's valve covers with an adjustable wrench. The condensing unit, the outside air conditioning unit, has brass service valves where the refrigerant lines enter the unit.
Twist the hoses from a refrigerant manifold gauge to the condenser's service valves. The left hose should connect to the service valve found on the thick refrigerant line and the right hose to the service valve found on the thin refrigerant line.
Identify the condensing unit's circuit breaker. Most units have an electrical disconnect box at the end of the wires that lead from the condenser; other units will have a circuit breaker marked "air conditioner" in either the circuit breaker box or in the electrical service box located at the electric meter. Leave the circuit breaker on for now; the unit will need the power disconnected quickly after evacuation.
Turn the air conditioning unit on at the thermostat.
Close the liquid-line refrigerant service valve with an Allen wrench. The Allen wrench will fit into the hex-shaped hole in the top of the service valve. Turn the wrench until the valve seats firmly.
Watch the manifold's left gauge. Once the gauge has a vacuum reading, close the large service valve with an Allen wrench. The Allen wrench will fit into the hex-shaped hole in the top of the service valve. Seat the valve firmly.
Quickly turn off the electricity to the condensing unit.
Heat the solder joints on the refrigeration dryer with an oxygen/acetylene torch.
Grab the refrigeration dryer with pliers and remove it from the refrigeration line.
Allow the refrigeration line to cool.
Slide the new refrigeration dryer onto the refrigeration lines. Arrows on the dryer will point in the refrigerant flow's direction. The liquid line, the thin line, flows from the condensing unit toward the inside unit and the suction line flows from the inside unit toward the condensing unit.
Solder the refrigerant dryers to the refrigerant lines using a silver-based solder. Heat each joint on one side and touch the silver-based solder to the far side. The solder will flow to the heat.
Connect a vacuum pump to the manifold's middle hose. Turn the vacuum pump on until the left manifold gauge reads about 29 inches of mercury. Hold the vacuum for one hour. If at the end of an hour the gauge reading has not changed, then continue. If the gauge reading has changed, then charge the refrigerant lines with nitrogen and find the leak with soapy water. Re-solder the leaking joint and retest.
Open the liquid-line service valve with the Allen wrench.
Open the suction-line service valve with an Allen wrench.
Turn the condensing unit's electricity on and remove the manifold's hoses.