A medicament delivery system configured to reduce the risk of inadvertently injecting medicament directly into a patient during a refill procedure via an external refilling apparatus. The medicament delivery system may include an implantable medical pump having a refillable medicament reservoir accessible via an access port, wherein the refillable medicament reservoir includes a flexible diaphragm having a nonlinear spring rate configured to create a relative vacuum within a medicament chamber when medicament is expelled from the medicament chamber beyond a predefined threshold, so as to spontaneously draw a quantity of medicament into the medicament chamber during the refill procedure to reestablish the predefined threshold, thereby confirming proper placement of a portion of the external refilling apparatus into the access port.