A treatment head for treating a keg's interior includes a treatment-head housing against which the keg is sealed by having the keg fitting pressed thereon and a lifter that is immovable relative to the treatment-head housing. In response to axial relative movement with the keg fitting, the lifter opens the keg fitting when the keg is sealed against the pressure element, thereby establishing a return gas-path through the treatment head, through a deformable seal and into the keg's interior. This relative movement is carried out against a resetting force. The deformable sleeve is a sleeve-shaped seal that surrounds a space that surrounds at least a portion of an axial extent of the treatment-head housing. A top end of the sleeve connects to the treatment-head housing and a bottom end of the sleeve connects to the pressure element.