Unscrewing the plunger can be tough, they are held in place by a small nylon lock tab molded around the threads in a small spot. If you can’t turn it by hand (loosening is counter clockwise) turn it with a small wrench. If it’s a round head try grabbing it with some smooth jawed pliers insulated by rubber/cloth/leather, etc. to avoid gauging the brass. Once the plunger is screwed all the way out, screw it back into place a couple threads and leave it in for now.

*This pic shows the small copper wire you don't want to break.