water ballast valve LS6c/LS7

Hi,

I'm struggling to elimitate water ballast leaks in a LS-7 after it has not been used for years. The wing valves seem to be one of the challenges. They leak regardless of the force applied to the actuator.

I assume the valves are the same as in LS6c (and not available as spare part any more)
Here is a picture of the valve https://goo.gl/photos/CvYt9SPy9efS8QR56

I couldn't find a way to dissasemble the valve to be able to inspect or replace the gasket. From one angle it's kind of possible to see into the valve, and it looks like it's just a metal ring pressing against plastic pipe with a tiny overlap. So no gasket?

Any idea how to fix the leak? Advice would be greatly appreciated, thanks!

Tibor