Abstract: A hydraulic cylinder monitoring apparatus of a type having a tube with a first end and a second end and a removable first cap member for selectively closing the first end of the tube, the first cap member having a first opening therein. A second cap member is welded to the other end of said tube, the second cap having a second opening therein. A piston is sealingly disposed within the tube for movement between a first and a second position between the first and second ends of the tube. A rod is operatively attached to the piston and extends from the piston and through the first opening in the first cap member whereby the piston and rod can move together between the first and second positions. A first hydraulic fluid port extends into the tube adjacent said one end of the tube. A second hydraulic fluid port extends into the tube adjacent the second end of the tube. A supplemental cap is removably attached to the second cap member, the supplemental cap having a passageway therein.