They are an old technology now and have been the bane of many a cycle mechanic. They are a closed system and the thumbwheel is a volume adjuster to take account of fluid expansion due to heat or whatever, you might even consider it an early form of bite point adjustment.
The problem you have is most likely to be too much fluid in the system, if you could turn the wheel no problem, as you cant just let some fluid out of the bleed nipple at the caliper end ( undo , gently squeeze lever a little and hold, do up) or the screw on the reservoir. A little at a time or you'll end up with no fluid and having to bleed the things
The wheels seize up for a past time, we have had therm in where the owner has taken pump pliers to them in order to shift the things. This i snot a good idea! You can or could get repair kits but I wouldn't bother. Get some replacement brakes from somewhere and fit them instead.