I remember when I was first starting out as a mechanic's apprentice in about '77. We worked on lots of these cars, and vacuum diaphragms of that vintage (really any Bosch vacuum diaphragm of the time) were a common failure. Easy way to check it... stick a vacuum hose on it, and suck on the end. You should be able to move the arm with your own suction. If it doesn't move, the breaker plate may be seized (not too common, as I recall), or the diaphragm is bad. Frequently with a bad diaphragm you can still suck it hard enough to move it. If you plug the end of the hose with your finger or your tongue the capsule should hold it's drawn-in position essentially indefinitely. If the diaphragm is bad it will slowly release over a couple of seconds.

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1987 W201 190D