Avid kits were supplied from the factory with Curtis drain valves - one for each wing fuel tank, and one for the aluminum header tank behind the seats. These valves have a rubber valve seal that will leak and fail over time. It is recommended that the valves be replaced rather than repaired. It may be tempting to try to install an O-ring in place of the rubber valve seal. This can be a temporary solution to get an airplane home, however it is not to be relied upon. New valves should be installed with sealant such as EZTurn (FuelLube).