i agree with lantraxco that the piston part of the adjuster is possibly stuck or cocked. had it happen on a cat 390D where the rod mushroomed or stepped or something inside the barrel. not entirely sure.
we failed to pull it apart with a 100ton press.. had to get a new one lol. there are two fittings, one with the nipple and one for slackening off, guys on site only ever checked the blank one to make tyr stop it leaking. turned out the grease nipple valve (exact same design as the release valve save for the nipple) was loose and was the reason for the leak. elementary, but nobody checked it.
what tonnage is a jd490?