A medical instrument includes a housing with a passage through which a catheter is received. A valve body is mounted in the passage, and, as the catheter penetrates through the valve body, the valve body conforms to the outer wall of the catheter to maintain a fluid tight seal between the valve body and the catheter. When the valve body is unstressed before being mounted in the passage, the valve body has a first planar dimension and a second planar dimension that is less than the first planar dimension.
