The invention relates to a sealing system for a closing flap on a through-opening of a device, which along with other similar closing flaps lying parallel to each other in a sealing plane when other devices are in a coupled position so interacts with the other flaps that the insides of both devices can be joined and the individual closing flap when closed provides a seal in relation to a pertaining through-opening along its perimeter. At least one of the two interacting closing flaps (22, 23) has a slit along its perimeter (24) which is bounded on both sides by annular shaped legs (26, 28). The slit (24) is bounded on the inside of each device (20, 21) by the leg (26) when in closed position. The leg (26) extends close to the sealing surface (27) of the through opening and the other leg (28) is radially reduced in comparison with the first one. An elastic bead (29) is located on the seal in an area which is radially outside the shortened leg (28). Said bead axially overlaps the shortened leg and rises at least partially above the sealing plane (34) between the closing flaps when the devices (20, 21) are in an uncoupled position. A pressure medium can be applied to the seal from the inside.