Abstract: A self-priming centrifugal pump including a priming piston disposed in a priming chamber which communicates with the pumping chamber to receive fluid therefrom. During priming, the priming piston is driven by the impeller shaft through an eccentric to discharge such fluid from the priming chamber. During normal operation of the pump, that is after the pump has been successfully primed, the priming piston is retained out of driven relationship with the eccentric by fluid supplied from the pumping chamber, thereby preventing the priming piston from taking power from the pump during such normal operation.