Anytime the A/C system is open to the environment you should replace the drier. My W124 has a separate (electric) vacuum pump for the door locks, but that's it. I'd replace the vacuum elements that control the heater door flaps/vents since you alreayd have the dash off. Or, at a minimu, the ones that are impossible to get to without taking most of the car apart. When my mechanic does an evap he does this as standard practice. He usually finds 1-2 that don't work anyway.

HTH,

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Brian Toscano