Abstract: A piston pump comprising a cylindrical sleeve member, a tetrafluoroethylene plunger head contained within the sleeve member, at least one O-ring contained within a circular recess concentric with and extending axially from the lower transverse face of and into the plunger head, and a piston rod having one end concentric with and rigidly mounted to this lower transverse face. Also, a pressure cap having a cylindrical uppermost portion dimensioned to fit into the circular recess of the plunger head, is adjustably mounted upon the piston rod, for axially compressing the O-ring(s), causing the O-ring(s) to exert a radial force against and effect a seal of a portion of the circular side walls of the plunger head against the opposing interior wall of the sleeve.