The application is directed to portable infusion system that may be used e.g. for infusing a material such as medicament e.g. insulin into a body in need thereof, said system comprises a delivery mechanism (132) including a bore (144) within a pump body (236) of the delivery mechanism, a spool (156) slidingly disposed in the bore forming a collapsible first volume (244) which is configured to communicate with a reservoir inlet port (138) and dispense port (142) of the bore independent of the reservoir inlet port, and a vent second volume (246) isolated from the collapsible first volume, the vent second volume configured to be moveable between a position that allows the vent inlet port (146) to communicate with a vent outlet port (148) and a position where the vent inlet port and vent outlet port are isolated from each other.
