There's a bellows and a vent on the pedal/cabin side. Probably has dust in it. There are also bushing inserts on the pedal bolts that are made of plastic, a replacement or a good cleaning/greasing can go a long way, but it can be a lot of work. The booster itself is a pressure differential with vacuum and gas filled chambers. Not much in the way of moving parts, more likely that the membrane will break than anything else failing on it.