If you have milk in the tubing it means something is damaged, or use is incorrect. While it is not as common, the use of an incompatible pump will cause milk to back up. Use of non-standard Freemie parts are also known to cause this problem.
If milk has entered the tubing, clean the soiled tubing immediately by rinsing with hot water. Shake off excess water and hang to dry. Use only when completely dry. It can take tubing a few days to dry. Replace soiled tubing that won't come clean. Note that water spots can look like dried milk. If in doubt, replace. Inspect the Y-connector part for dried milk and wash in hot soapy water and rinse well if this happens.
In most cases, milk backing into the tubing means that one or both cup barrier parts are damaged. The barrier is the soft silicone tongue-shaped part that mounts onto the breast flange. Always inspect barriers before/after each use for any rips, tears, or cracks. This important part protects your pump from milk, and will damage your pump if used when damaged. Replacement barriers are available here.
Watch this video to see how to inspect your barrier part:
Barriers are durable but can get damaged. Check barriers under a bright light. Uncurl the edges and squeeze open to look for any damage. Also check for any dried milk or obstruction that could keep the barrier from sealing against the breast flanges. Replace immediately if damage is found. Do not use your pump with a damaged barrier part.
When assembling cups mount the barriers onto the backs of the breast flanges, and then cover with the valve base/valve assembly.
IMPORTANT: Inserting barriers into the valve base first, then assembling to the breast flange, could prevent the barrier from mounting properly, and will either cause no suction or milk to back up into the tubing. Insertion may also damage your barriers.
Watch these two videos to see how to assemble your barrier and valve base parts step by step:
Step-by-step cup assembly directions:
Read more about how to care for your Freemie parts.