In the 149 the complete piston unit is screwed into the barrel from the rear. If that joint works loose, turning the knob to fill the pen no longer drives the piston forward but unscrews the whole unit out of the barrel. The pen is not faulty on that account; the unit simply has to be seated firmly again.
Doing that by hand without leaving marks is all but impossible. There are purpose-made tools that key precisely into the recesses of the piston unit and allow it to be tightened evenly. They are available from specialist pen dealers and from suppliers who concentrate on restoration, Penboard among them; wrenches exist for virtually every older and newer Meisterstück series, along with tools for pulling the nib and feed. The important things are to choose the size that matches the model and to work without force, since the barrel and its thread are made of precious resin and can crack if overtightened. Anyone who does not feel confident is better off sending the pen to a specialist workshop or to the manufacturer's service department.