a relief valve for a hydraulic piston consists of a tubular mid - section with closed ends and circular stops at each end . the tubular center section has cross ports at each end that run perpendicularly to the axis of the valve . the cross parts are situated as close to the end stops as possible . the cross ports intersect the hollow center of the spool and allow hydraulic oil to flow through the piston via the passageway in the center of the valve when the valve is in the open position .