A constant volume metering chamber and valve assembly for use with a pressurized fluid supply containing a fluid is described for use in a power fastening tool. The assembly includes a first spring-biased valve and a second spring-biased valve. There is a metering chamber having an inlet and an outlet, so that the first spring-biased valve controls the inlet and the second spring-biased valve controls the outlet. An actuator assembly operates causes the first and second spring-biased valves to move in either a first valve sequence or a second valve sequence. The first valve sequence includes engaging the first spring-biased valve with the inlet, preventing flow of the fluid from the fluid supply into the metering chamber, followed by disengaging the second spring-biased valve from the outlet, allowing flow of the fluid from the metering chamber. The second valve sequence includes engaging the second spring-biased valve the said outlet, preventing flow of the fluid from the metering chamber, followed by disengaging the first spring-biased valve from the inlet, allowing flow of the fluid from the fluid supply into the metering chamber, such that fluid is prevented from flowing freely from the pressurized fluid supply through the inlet and the outlet of the metering chamber.