A piston assembly is for a fluid delivery device, preferably a lubricant injector, the injector having a measuring chamber defined by an inner cylindrical surface and an axial end face, a flow port through the end face, and a central axis extending through the chamber. The piston assembly includes a piston body disposeable within the chamber so as to be linearly displaceable along the central axis. The piston body includes a cylindrical head, the head having first and second axial ends and an outer circumferential surface, and a rod extending axially from the second axial end of the piston head. A seal is coupled with the first axial end of the piston head and has an annular lip sealingly engageable with the chamber inner cylindrical surface and an axial end engageable with the chamber end face.