Thanks Chuck. I read your text just now. I did check all fingers before beginning work and also during the development, when I saw the leakage. This is most worrying to me because the tank looked quite well closed and everything looked in order. I would almost prefer if I had a broken finger or if the tank always leaked. The way it is now, the tank looks fine when it leaks and when it does not.
I think the plastic begins being worn out and if it is not pressed with a lot of energy it will not be really tight.
Besides, when I pour in 140 ml I often get back 135 ml. I would understand this to happen the first time, when the tank is dry. But it goes on and on. I often recover "just a bit less" than what I pour in, even on the tenth attempt. That makes me suspect that the tank is actually worn and not any more reliable.
I want to try with presoak because at least that would give me peace of mind regarding water tightness.
In the meanwhile I should get another tank.