1991 560SEL AC Recirc Motor Vacuum leak

I have a 1991 560SEL with 68,000 miles. Great car- had it since new. The quality surpasses the new ones I think. Today I noticed that on a long bridge going up the AC recirculate door
closed ( stopped recirculating ). This would suggest a vacuum leak or weak vacuum motor or bad hose. Does this car have a vacuum reservoir or some sort of check valve system for the AC system. Thanks Barry
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