Over time, moisture contamination in an A/C system will form corrosive acids that attack metal surfaces and break down the lubricating qualities of the compressor oil. The result is a dark-colored sludge called "Black Death" that can gum up the orifice tube or expansion valve and ruin the compressor.

How does moisture get in the system? Humidity in the air can slowly infiltrate microscopic pores in seals and hoses over time (we're talking years, in most cases). But a refrigerant leak due to a bad seal, hose or hose connection, or a pinhole leak in the evaporator or condenser can allow air and moisture to seep in as refrigerant leaks out. Moisture can also contaminate the system very quickly if the A/C system is discharged and a hose is disconnected and left open for more than a few hours (which often occurs when repairs are made). The higher the relative humidity, the faster the rate of contamination when the system is left open.

Another source of moisture contamination can be bad refrigerant. Virgin refrigerant contains no air or moisture. Neither does recycled refrigerant, provided it is recycled properly and the recycling equipment is maintained and working correctly. But if there's a problem with the recycling equipment, the filters haven't been replaced or the storage tanks have not been purged, residual moisture from one vehicle can be passed to the next via the recycled refrigerant. Consequently, if the A/C syst...