An expression kit (2) for use in a breast pump device (1) for extracting breast milk from a human breast comprises a breast pump body (20) having a first pressure chamber (21), a second pressure chamber (22), and a membrane portion (5) separating the first and the second pressure chamber (21, 22) from each other. The membrane portion (5) is configured to form a barrier between the first and the second pressure chamber (21, 22) for preventing leakage of human breast milk from the second pressure chamber (22) to the first pressure chamber (21), yet to allow for air exchange between the first and the second pressure chamber (21, 22), wherein the membrane portion (5) is hydrophobic and has the shape of a solid sheet being provided with holes (50) which are configured for rendering the sheet permeable to air and impermeable to liquid.
