From the NMC Newsletter "Udder Topics", September 2004

Improper drainage is a common source of bacterial contamination in milking systems. Pooling of fluids within the milking system can also lead to mixing of cleaning solutions which may reduce their effectiveness. All parts of the milking system (both sanitary and non-sanitary) should drain when the system is shut off. When troubleshooting cleaning problems, be sure to inspect the system for any pipes, hoses, fittings and equipment that do not drain after shutting down the system.

Source: NMC publication “Troubleshooting Cleaning Problems in Milking Systems” (2003).