I'd suggest that you call HWH and describe the problem and let them help you troubleshoot and advise exactly what has to be done. I remember reading that some of their jacks built before 2009 or thereabouts, had piston problems that could not be solved other than replacing or rebuilding the jack.
HWH can advise you on if it is something that you can fix yourself or if you have to have the entire jack replaced or rebuilt.
800-321-3494 or 563-724-3396
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