Inside the drier is a pipe that leads to desiccant, similar to that little white package found in a new ring and pinion box or bag of beef jerky. However, this desiccant pack is about 11/4 inches thick and filters out debris in addition to pulling moisture from the system. The liquids pass through the desiccant to the bottom where a tube picks them back up and sends them to the expansion valve. If the drier is beyond its useful life, you might see evidence in the form of the expansion valve freezing or the compressor running too hot (oil and water don't mix). If moisture gets into the system it can react with the R-12 or R-134A and form an acid that will eat the aluminum parts from the inside. For that reason, it is strongly suggested to replace the drier every time you open the system, and to keep it capped until just before you are ready to pull vacuum on the system.