Disclosed herein is a fluid infusion device of the type that delivers medication fluid to the body of a patient. The device includes or cooperates with a fluid reservoir, and the device has a sealing assembly to receive and form a fluid seal with the fluid reservoir. A retractable sealing element surrounding a hollow fluid delivery needle may be used to seal a port of the fluid reservoir. The port may include a pressure vent that is sealed by the retractable sealing element. In one variation, the reservoir includes a moving valve sleeve that holds a septum. The septum moves to allow the reservoir to vent, and to form a seal with the port when the needle pierces the septum. In another variation, the device includes a needleless sealing assembly. In yet other variations, the device uses a needled fluid reservoir.