The behaviour described — air emerging from the nib on drawing up, barely ink absorption — is a classic symptom of a clogged feed. Even a single filling with dried black ink can suffice to clog the capillaries so much that feed and piston have to be exchanged. As a first measure, long, multi-day soaking with subsequent flushing is to be tried. If that brings no success, factory repair at Montblanc is unavoidable; they routinely carry out such complete cleanings.
Related pages on fountainpen.de:
• Montblanc Meisterstück 149 (Grand Format): https://www.fountainpen.de/c-montblanc-meisterstueck-149-en.htm