Grease Lubricators are essential for any effective preventative or predictive maintenance program. Installing them will help ensure the right lubricant, in the right amount, and in the right frequency is successfully implemented as the foundation of you predictive/preventive program.
Air pressure can build up in a bearing housing for several reasons. One, is that the fill port is closed with a plug. Additionally, the shaft seals are not exchanging any air. Either they are tight lip seals, magnetic seals, or could be labyrinth seals that become defective due to installation.