A method is provided for controlling the return phase of a blood separation procedure having a draw phase in which blood is drawn through an access or donor line, is separated, and a fluid, such as a cellular concentrate, is flowed into a reservoir, and a return phase in which fluid is flowed from the reservoir through the donor line. The method comprises flowing fluid from the reservoir through the donor line in a steady state return monitoring the donor line for the presence of air and, upon detecting air in the donor line, determining the amount of fluid remaining in the reservoir. Then, either i) the donor line is purged, if the amount of fluid remaining in the reservoir exceeds a predetermined amount, and the steady state return of fluid from the reservoir through the donor line is resumed or ii) if the amount of fluid remaining in the reservoir does not exceed the predetermined amount, the return phase is ended by terminating the flow of fluid from the reservoir through the donor line.
