The presence of residual water in the upper chamber of a Berkey water filter system is a common occurrence and not a cause for concern. The water needs to pass through fine micropores within the Black Berkey Elements to move from the upper chamber to the lower chamber. The level of water in the upper chamber affects the pressure available to push the water through the micropores. As a result, the water purification process is faster when the chamber is full compared to when it is partially full. During normal use, the residual water mixes with incoming water and is purified in each cycle. However, it is advisable to empty the upper chamber before discontinuing the use of the system for an extended period of time, such as during a vacation.