Thank you both for your input.
I already know that pump is working normally by feeding it from outside the airplane.
I know that main tank is also ok as i was able to access it and checked/changed tubes inside.
Problem definitely relies in saddle tanks and vent line.
Issue is that those tanks were glued to the airframe by initial builder and stoppers are not accessible....
I am not comfortable with the tubing inside the saddle tanks to start with.
I could eventually change the vent lines to larger ones, but unsure if this would solve completely the problem.