A method of maintaining a locking fluid in an implanted catheter including positioning a catheter assembly in a patient, the catheter assembly including a first lumen having a closed first distal end and a first proximal end, and a second lumen having a closed second distal end and a second proximal end and injecting the locking fluid into the first and second proximal ends at a low pressure such that the locking fluid enters each lumen without opening the respective closed distal end. A method of withdrawing the locking fluid is also provided.