What can cause massive post nasal drip causing pneumonia when an x-ray shows sinuses clear?

See an ENT: Imaging studies of the sinuses may not always reflect what is present. Even ct scans need to be appropriately directed. This seems to be a serious problem and you should be seen by an experienced ENT specialist.

It doesn't: That would be a mechanical issue with how the body protects the airway. The wet stuff is supposed to go in your swallow tube & only dry air into your lung. However, they share some of the same structures, with a valve, the epiglottis there to protect the airway. If the system isn't working, any wet debris from above can enter the lung & cause pneumonia. Needs an ENT evaluation.