I bought a Wanner grease gun in my teens, it can apparently generate very high pressures and as such the connector locks on to the grease nipple to the extent that there is a bleed screw that can be used to depressurise the feed line if required, to release the connectors grip on the nipple if required.. I took it for granted that most grease guns would operate similarly... I was obviously wrong, yet again..

Every day has potential to be a school day. Thanks.