Ok, I had a look to my Gotoh 1996T tremolo and probably I understood the issue and how to easily fix it.
Old MM Floyd tremolos are basically Gotoh 1996T tremolos.
The tremolo arm has a threaded seat on the bottom of its part that you put in the tremolo's seat and the tremolo's seat has a hole in the bottom: a passing screw, screwed in tremolo arm's threaded seat, secures it into seat through the hole, so that tremolo arm cannot pop out.
Then you can set tremolo arm tension to your taste by losening or tightening the other small screw on tremolo's seat top, where the clamp is.
If you remove the screw that secures tremolo arm to tremolo's seat, you can pull out and push in the arm whenever you want, since setting its tension in the clamp it's enough to avoid the arm popping out the tremolo.
So, tbone you were right.
Furthermore, that passing screw is completely useless to me (infact I don't have it).