The c-class cars use vacuum diaphrams to move the different a/c box doors, sounds as if at least one of these diaphrams is leaking vacuum. Also there is a vacuum storage tank & manifold that can leak. The vacuum control valve (a/c system) inside the car is the place to start.

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MERCEDES BENZ MASTER GUILD TECHNICIAN (6 TIMES)
ASE MASTER TECHNICIAN
27 YEARS DEALER M.B. Shopforeman
190E 2.3 ITS RACECAR
1986 190E 16V