Ian,
Your Nimbus 3 has internal springs that have long since
rusted off (part of the problem) and there is no practical
way to replace them because that involves opening up
the wing. I added stronger external springs that apply
more torque to the dump shaft (s) , one on the inboard
and another on the outboard, as I remember. Then I
would goop up the dump valves with chap-stick, insert
the 6mm tool and pull down with a half twist. That
seemed to do the trick. In real hot weather I would
use very heavy wax, the stuff that your toilet bowl
sits on.
JJ