I have some air coming from I don't know where yet

it is a 1.6 TD from 87, some no pre heating on the fuel filter.
I have by passed the eberspraecher, so this is not where the issue comes from.
I have a question : When I disconnect the hose from the tank, only a few drop of fuel is going out..of course the tank is full, so I assume it should be a lot of fuel coming out...Any idea ?
Thanks for any tips,
Lionel