A blood separation procedure having a draw phase in which blood is drawn through a source access line, is separated and a fluid is flowed into a reservoir, and a return phase in which fluid is flowed from the reservoir through the source access line, wherein the blood separation procedure comprises a method for controlling the return phase of the procedure, the method comprising flowing fluid from the reservoir through the source access line； monitoring the source access line for the presence of air； and upon detecting air in the source access line, determining the amount of fluid remaining in the reservoir and either purging the donor line if the amount of fluid remaining in the reservoir exceeds a predetermined amount and resume flowing fluid from the reservoir through the source access line, or ending the return phase by terminating flowing fluid from the reservoir through the source access line if the amount of fluid in the reservoir does not exceed the predetermined amount.
