Provided in a housing of the warning device is a conical guide element which has a bore running in the direction of its longitudinal axis. A tripping pin provided with a stop is displaceably arranged in the housing against the force of a spring, and in fact in such a way that a part of the tripping pin can be pushed out of the bore in the guide element and out of the housing. The tripping pin is held against the force of the spring by a locking device. The locking device consists of an expanding body which is provided with projections and on the one hand is supported with its expanding elements on the conical circumferential surface of the guide element and on the other hand projects slightly from the housing through an opening in the housing. The warning device is fastened in an opening of the cylinder housing of a diaphragm cylinder in the area of the diaphragm. If the diaphragm is overstretched, it bears on the locking device and thus unlocks the tripping pin. The tripping pin can be designed as an optical indicating device and/or can have electrical contacts which are connected to an electric indicating device. <IMAGE>
