The 'self bailer' or 'venturi' is situated at the rear of the cockpit where it sucks water from the cockpit above a certain boat speed. The venturi is opened and closed by the o rings shutting the vavlue when the cockpit bung is pushed in. Through being stretched over time, the o rings get worn out and need replacing to maintain the functionality of the bailer.
Replacing the o rings will allow you to shut your bailer in light winds, and keep your boat light. An alternative to the o rings are the bailer springs, which last much longer.