A reciprocating well pump assembly has a barrel and a piston reciprocally carried within the barrel between up stroke and down stroke positions. A travelling valve moves with the piston and has a closed position during the up stroke an open position during the down stroke. A relief port extends from a bore of the piston below the travelling valve into the barrel. A valve sleeve mounted around the body of the travelling valve is axially movable between a closed position, closing the tubular body relief port, and an open position, communicating well fluid in the barrel with the well fluid in the bore of the piston. When nearing the top of the stroke, the valve sleeve abuts a stop in the barrel. Continued upward movement of the piston causes the valve sleeve to open the relief port.