According to the service guide, water must enter the body of a Boheme neither from the front nor from the rear; only rinsing the nib is officially allowed. A thorough cleaning is however needed especially when black ink is used (which clogs quickly) or when the ink colour is to be changed. In boutiques Montblanc uses a special glass tube with a rubber bulb, fitted in place of the cartridge; this tool is not sold openly and is comparatively expensive at around 30 euros. A pragmatic solution is to open an old Montblanc cartridge at the rear with a knife (without shortening it too much), clean it, and attach a Montblanc piston converter; the new converters with a front thread sit particularly firmly. The empty cartridge is fitted into the Boheme, the converter attached, and the nib turned out, which retracts the converter together with it. By dipping the nib into water and operating the converter, water is drawn through the feed until the ink lightens visibly. Note that the cartridge must not be cut too short, otherwise the metal ring of the converter does not fit into the body; in an emergency a second empty cartridge can serve as an extension.
Related pages on fountainpen.de:
• Montblanc Bohème — model overview: https://www.fountainpen.de/boheme-en.htm