If it is making a clacking noise, change it immediately. That happens when the ball bearings fail so that arm is dragging on "roller coaster ramp". In a few hundred miles, the arm will be cut thru and pieces slide down into the timing chain, most likely causing massive engine damage. I caught mine when the arm was cut half thru after ~100 miles driving by a family member. This failure can happen with no noticeable loss of vacuum.

Otherwise, do like Dan says in post #3. Step one is disconnect the main vac line from the pump and connect to just a vacuum gage. If you measure >15"Hg vaccum w/ engine idling, your pump sounds fine. If the shop didn't do so, run away and find a competent shop that can do obvious diagnostics before stating, "you need ...".

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans