As suggested, first take out the button (pull and rock it) then the pressure bar (gently.) Then if the section doesn't want to unscrew, it means the sac is twisting inside the barrel. You may have to force the section to twist, and cement a new sac to the service. New sacs are available (Pen Sac Comp. in CA), and should fit next to the pressure bar snuggly, probably a #18 or #20 sac.