oreo,
"Generally" when the needle bar keeps reciprocating after the stop motion knob is released it's because of old gummed up oil between the hand wheel and main shaft. You'll need to remove the hand wheel, and thoroughly clean both the bore of the wheel and the shaft. Make sure there is no burrs or dents or gouges on the shaft or inside the wheel, ( I've had to polish out a few although why they are there is beyond me ) then put one drop of oil on the shaft. Just enough to prevent bare metal against bare metal. That should take care of the problem.
I don't know anything about the break. Sorry I can't help there.
Joe