1996 GMC 4X4 not staying engaged

1996 gmc k 1500 front diff not staying engaged. Took out actuator and applied 12 volts. Did not extend at first but did eventually. Only partially at first then further each time. Power at act plug (12.25 volts). If 12 volts applied actuator heats up and extends is normal operation should I be able to push back in easily?If so how does diff stay in? Diff is locked by spring press isnt it? Spring press in actuator port is stronger than actuator pressure after extension. Is actuator supposed to stay extended with enough pressure to keep diff locked? If so how does diff unlock when shifting out of 4 hi or 4 lo to 2 hi?I guess im asking how this is actually supposed to work. Sorry but I like to know how something operates not just how to fix it. Would prefer not to just start replacing parts ie actuator if not problem. Trans case locks in and stays in both hi and lo. Has push buttons on dash. Also where to find axle code. Thnx