This is not always obvious to the naked eye unless you hit the wood with the fleshy side of your fist. Then you will hear a 'slapping' type sound. Cracks also develope at the seams where various pieces are glued together. This actually happens quite a bit in 1920's units. It's due to their age.
All wood continues to dry and shrink for many, many years.
At some point, if not well sealed, the seams split open and let the air 'in'. Since the unit stores vacuum, sealing these types of cracks with silicon or other such sealers is a reasonable 'quick fix'. However, it is better to remove, clean, reglue and clamp this type of damage.