As far as I remember, a lot of the parts on them are Zamak (zinc alloy casting). The point of connection between the trigger and piston sear is probably worn and needs a little re-shaping with a file to make it more positive. Personally I would bin it as I hate them, but as you have a sentimental connection you should have a go as they have a very simple design. ggggr on here will probably help you.

Baz