The plug-in piece consists of a connecting sleeve (3) made of strong material and enclosed at least on the end sides by elastomeric sealing rings (4,5) placed close to the inner wall of the hollow bodies. At least one sealing ring protrudes axially beyond the corresponding end side of the connecting sleeve. the protruding part (8,9) is in the form of a sealing lip (10,11) sprung radially outwards and which when exposed to excess pressure is placed inside the plug-in piece and rests tightly against the inner wall (6,7) of the hollow body (1,2) enclosing the outside of the end wall.