It is important to vacuum an R-22 air conditioning system properly before charging the system. Pulling a proper vacuum ensures that the moisture and non-condensable gases are removed. The moisture mixes with the oil and creates an acid that eats away the insulation on the motor windings as well as causing corrosion on other parts of the compressor and shortening its life. The moisture can also collect at the point of restriction, clogging it up. The non-condensable gases can create high head pressure causing the compressor to work harder, again shortening the life of the compressor.
Connect the manifold gauges, making sure no pressure is present in the system. Screw the high side hose (the red hose) to the Schrader valve on the high side of the system. The high side is the smaller diameter copper line. Screw the low side hose to the Schrader valve on the low side, which is the larger diameter copper line.
Screw the middle yellow hose of the manifold to the micron gauge, which is connected to the vacuum pump. Turn the two handles on the sides of the gauge manifold counterclockwise to open the manifold valves.
Turn the vacuum pump on. The dials on the gauges will slowly start moving to the left into a vacuum. The vacuum pump should be left on until the micron gauge reads at least 400 microns.
Close the valves on the manifold gauge set by turning the valve handles clockwise. Next, turn the vacuum pump off. Let the unit sit for 15 or 20 minutes to see if the unit holds the vacuum. If it is loosing the vacuum, this indicates a leak that must be repaired. If the unit holds the vacuum, remove the yellow hose from the micron gauge.
Connect the yellow hose to the R22 container. To break the vacuum, open the valve on the R22 container. Purge the hose by slightly twisting the connection between the yellow hose and the manifold gauge until a small amount of refrigerant comes out. Then tighten the fitting again. This eliminates the air that gets in the hose when it is removed from the micron gauge. Open the valves on the manifold gauge set and allow the pressures to equalize. The air conditioner is now ready to be charged.