The S4 cup routes all of its suction through one small silicone assembly: the duckbill valve that gates milk into the cup and the airbag that keeps liquid out of the pump unit. This is that part, original and exactly moulded, in a one-piece pack.
The duckbill and airbag do two jobs at once — maintain the sealed vacuum that creates suction, and guide milk downward while blocking it from the tubing and motor. When either fatigues, both jobs suffer at the same time.
Phanpy’s guidance is plain: replace this part roughly every two to three months, sooner under heavy daily use. Suction that feels weaker is usually this membrane asking for retirement, not a pump failing.
100% food-grade, BPA-free silicone with nothing questionable in the formula — the part touches milk on every cycle, and it is built like something that touches milk.
Warm soapy water after each session is the daily routine; steam, boiling water or a microwave steam bag handle sterilizing. The silicone is rated for all of it through its full service life.
A 20 mm tunnel for mothers the standard flange fits too loosely — the two-minute fix for pinch-free pumping.
