They are completely sealed. That's why they have a pressure-relief plug or can scoring.
That's also why they explode when they fail.
The manufacturers recommend that the caps be re-formed (see post #3) if they've not been powered up for six months.
Novatron recommends operating their packs at least 30 minutes every four weeks.
They recommend re-forming the capacitors if the pack has sat unused for longer periods.