Blood processing systems and methods rotate a processing chamber on a rotating element. The processing chamber includes a first compartment and a second compartment. Blood is conveyed into the first compartment for centrifugal separation into components. A liquid free of blood occupies the second compartment to counter-balance the first compartment during rotation on the rotating element. In one embodiment, the second compartment is served by a single fluid flow access. Prior to use, the single access is coupled to tubing, through which a vacuum is drawn to remove air from the second compartment. While the vacuum exists, communication is opened between the tubing and a source of liquid. The vacuum draws the liquid into the second compartment through the single access, thereby priming the second compartment for use.