The standard dampers are a twin tube design, the central tube is full or oil which the piston moves up and down inside. It is the oil passing through small passages in the piston that convert the movement into heat. The outer tube provides a reserviour for extra oil and some trapper air. When the damper is compressed the rod displaces oil out of the central tube into the outer and the air is compressed
If the damper has not been used for a little while then the air can get inside the central cylinder. Initially you can hear the air as you extend/compress the damper, but on a damper in good order the first few stokes will purge the air out of the central tube and it will regain a smooth firm action.
If the air will not purged after several stokes, the damper is springy or is not smooth then its probably worn out.
