A trocar is adapted to form a seal around a surgical instrument and includes a cannula having an axis extending between a proximal end and a distal end. A seal housing disposed at the proximal end has a proximal wall with a hole extending into the housing. A septum valve disposed within the housing has a normal state when the instrument is absent from the trocar, and a stretched state when the instrument is being withdrawn from the trocar. A spacer extending between the proximal wall and the septum valve, engages the valve at a particular position which separates the valve into an inner portion, which is adapted to form a seal with the instrument and an outer portion, which is adapted to form a seal with the housing. The spacer has an axial length sufficient to space the particular position of the septum wall from the hole in the proximal wall a distance greater than the radial length of the inner portions of the valve in the stretched state. The septum valve can be configured to provide an instrument insertion force less than an instrument withdrawal force.