I have a Filstar. There's a couple plastic tabs on the inside where the valve mechanism is. They are along the arc of where the valve turns. If you can manage to push both in at the same time you can open the valve. I've found that this does clean the tubes a bit, but I'm nervous about doing this too much for fear of breaking the thing.
I'm in the same boat, my tubes look dirty. I haven't had the will to break it down completely yet and try to clean them, the next step may be to try some of those long brushes and clean from above, without getting it into the tank water!
Good luck!