[drew1]

Like URY said most jacks are throwaway nowadays. Since you pump up OK, the balls & seats in the check valves are OK & the O ring on the pump piston is OK. The V ring on the piston holding the jack up is ptobably shot. The ID of the cylinder probably has a bad spot that tore the ring.

If you can hone the cylinder smooth the jack can be fixed with a kit from the manufacturer, the correct V rings , or a gland to replace the V ribngs with an O ring. UHMWPE is a good material for the gland since it can be split & slid over into the V ring groove & turned with close enough fit to provide a back up for an O ring.