I recently restored two swan leverless pens, one an early Gold filled pen and the other a 1930s 0160. The sac replacement is really straightforward, fully clean out old sac and fit new one with a good dusting of chalk.
The action of the system is usually described as 'twisting the sac' but I don't think that is actually what happens. When the button is turned the bar moves in such a way that the edge of the bar presses along the length of the centre of the sac pushing air out. When the button is tightened the bar returns to its original position, the sac is allowed to regain its shape and ink is pulled in. it is really very efficient, one turn is all that's needed.
The fact that the edge of the bar presses on the sac would, I suppose, be more likely to damage the sac than more conventional systems, perhaps this is why the system was abandoned.