The present invention discloses a control system for operating a vitrectomy probe and controlling a port size of the vitrectomy probe comprising: an output valve a first output port operable to communicate pneumatic pressure to the vitrectomy probe, the first output port coupled to the output valve a second output port operable to communicate pneumatic pressure to the vitrectomy probe, the second output port coupled to the output valve a first conduit coupled to the output valve and operable to communicate pneumatic pressure to the output valve a venting control valve a second conduit extending between the output valve and the venting control valve operable to conduct pneumatic pressure from the output valve to the venting control valve a first outlet coupled to the venting control valve and open to the atmosphere and a controller operable to: oscillate the output valve between a first position in which the first conduit is in communication with the first output port and the second conduit is in communication with the second outlet port, and a second position in which the first conduit is in communication with the second output port and the first output port is in communication with the second conduit, the output valve maintained at the first position and the second position for a selected time period move the venting control valve froma first position in which the second conduit communicates a first amount with the second outlet and a second position in which the second conduit communicates a second amount with the second outletwhen the output valve is in one of the first position or second position maintain the venting control valve in the second position for a first period of time less than the selected period of time when the output valve is in one of the first position or second position to definethe port size of the vitrectomy probe and move the exhaust control valve from the first position to the second position after the first period of time has elapsed.
