[DonVoigt]

Let it soak for several days in a thin penetrant oil- there are many, I use whatever is on sale.
Might take a week.
Gentle pressing of the plunger when it starts to move will work the new thin oil in and the old gunk out.

In the unlikely event it is a rust problem or soaking does not work, you will have to remove the pin. Though the pin really will not free up anything if you can't grasp the plunger with pliers- i.e. if it is pushed in flush with the end of the bar. May let a little more penetrant into the game.

The pin is smaller than any punch in my tool kit, so you will have to improvise, a small metal paper clip is the right size and if the small pin is not too "stuck" - you can press it out by hand.
If hand work doesn't do it, you will have to cut a very short length of clip, place the sear bar with pin over a hole in a flat metal support, and use a very small hammer to tap the improvised punch, I suggest holding the punch with needle nose pliers, it needs to be very short or it will bend.

Hope this helps.