A medication infusion pump is provided for substantially constant flow delivery of a selected medication from a medication chamber through a catheter to a patient. The medication infusion pump comprises a compact housing adapted for implantation into the body of a patient, wherein the housing defines a hollow interior forming a medication chamber. An inlet port on the housing permits transcutaneous access to and filling of the medication chamber within the housing. A flexible propellant reservoir occupies a portion of the medication chamber volume and is filled with a selected fluid propellant to apply a substantially constant pressure to the medication for medication delivery with substantially constant flow through an elongated capillary tube and the catheter for administration to the patient. The capillary tube is spooled within the housing at the perimeter thereof in association with an annular filter of large surface area to prevent impurities and particulate from entering and potentially clogging the capillary tube. The inlet port also includes a click plate for providing a positive feedback indication of proper registration between the inlet port and a refill needle for refilling the medication chamber.