Combined service brake and spring brake cylinder for brake systems of vehicles, in particular commercial vehicles, comprising a) a service brake piston or service brake diaphragm (25, 27), which is located in the service brake cylinder and actuable by a pressure fluid and which is operationally connected to a service brake piston rod projecting from the service brake cylinder and acting on actuating elements, b) a spring brake piston, which is located in the spring brake cylinder and actuable by an accumulator spring and which is bounded by a pressure chamber on one side and by a spring chamber accommodating the accumulator spring on the other side, the accumulator spring being supported on the spring brake piston on the one hand and on the base of the spring brake cylinder on the other hand, c) a partition between the service brake cylinder and the spring brake cylinder, through which a spring brake piston rod connected to the spring brake piston extends, d) a bayonet lock between the spring brake cylinder and the partition, which bayonet lock comprises locking elements projecting radially outwards at an end edge of the wall of the spring brake cylinder as well as radially inward-formed recess segments and corresponding radially outward-formed recess segments formed on an annular chamber connected to the partition and locking segments projecting radially inwards into the annular chamber, e) a base-side stop face in the annular chamber as an axial stop of the end edge of the wall of the spring brake cylinder, f) a ring seal arranged between the radially inward-oriented circumferential surface of the edge of the wall of the spring brake cylinder and the radially outward-oriented circumferential surface of the annular chamber, characterised in that g) a recess is provided in the base of the annular chamber for the accommodation of at least an end section of the ring seal, and h) end-face contact surfaces are provided on the locking elements of the spring brake cylinder for contacting the base-side stop face in the annular chamber, wherein i) at least the end section of the ring seal is clamped between the recess in the base of the annular chamber and the edge of the wall of the spring brake cylinder. a) a service brake piston or service brake diaphragm (25, 27), which is located in the service brake cylinder and actuable by a pressure fluid and which is operationally connected to a service brake piston rod projecting from the service brake cylinder and acting on actuating elements, b) a spring brake piston, which is located in the spring brake cylinder and actuable by an accumulator spring and which is bounded by a pressure chamber on one side and by a spring chamber accommodating the accumulator spring on the other side, the accumulator spring being supported on the spring brake piston on the one hand and on the base of the spring brake cylinder on the other hand, c) a partition between the service brake cylinder and the spring brake cylinder, through which a spring brake piston rod connected to the spring brake piston extends, d) a bayonet lock between the spring brake cylinder and the partition, which bayonet lock comprises locking elements projecting radially outwards at an end edge of the wall of the spring brake cylinder as well as radially inward-formed recess segments and corresponding radially outward-formed recess segments formed on an annular chamber connected to the partition and locking segments projecting radially inwards into the annular chamber, e) a base-side stop face in the annular chamber as an axial stop of the end edge of the wall of the spring brake cylinder, f) a ring seal arranged between the radially inward-oriented circumferential surface of the edge of the wall of the spring brake cylinder and the radially outward-oriented circumferential surface of the annular chamber, characterised in that g) a recess is provided in the base of the annular chamber for the accommodation of at least an end section of the ring seal, and h) end-face contact surfaces are provided on the locking elements of the spring brake cylinder for contacting the base-side stop face in the annular chamber, wherein i) at least the end section of the ring seal is clamped between the recess in the base of the annular chamber and the edge of the wall of the spring brake cylinder.