In one aspect, a brake pump assembly with a hydraulic block and a pumping element including a polymer piston that is received in a piston bore. A circumference of the polymer piston defines a high-pressure seal slideably engaging adjacent structure. The polymer piston may include a peripheral sealing lip projecting primarily longitudinally from the polymer piston for sealing engagement with the piston bore. In another aspect, a brake pump assembly with a hydraulic block, a straight bore polymer piston, a peripheral sealing lip projecting from a proximal portion of the polymer piston, and a elastomeric seal mounted to a distal portion of the polymer piston, where the peripheral sealing lip selectively acts to depressurize fluid accumulating between the peripheral sealing lip and elastomeric seal.