A fuel pump including a low pressure infeed leading to a pumping chamber, a pumping plunger for pressurizing fuel in the pumping chamber, an outlet valve for delivering pressurized fuel from the pumping chamber to a high pressure outlet line during pumping, and an overpressure relief valve connected between the high pressure outlet line and the pumping chamber. A ball type relief valve element is spring biased to close against the relief valve seat. The spring is in a spring chamber that is fluidly isolated from the pumping chamber while the valve element is closed, preferably in direct hydraulic communication with the low pressure infeed, thereby reducing dead volume during normal pumping. The spring can act on the valve element through a piston, and in an alternative embodiment, the piston can provide a spill path from the pumping chamber to the low pressure infeed.