Is this the result of a vacuum leak?

Car: 1995 e320

Today A/C stopped blowing cold (blows kind of luke-warm now) and there is a persistent hissing from inside the center vent when either of the two A/C buttons on the push button/climate control unit are pushed. When EC or Defogger are pushed, the hissing stops.

I also can hear the hissing inside the engine bay.

Everything else works as it should:

• The compressor engages and I see the Freon through the glass on the bottle/receiver.

• Hot air blows out the side vents when heater is on (normal)

• Cold (or now luke-warm) air is coming out the sides and center (normal, except for lack of cold air)

• The defogger blows out the top of the dash (normal).

In the heat of the afternoon, in Auto mode, the blower motor runs at the same speed as in High mode. Switching back and forth does not alter blower speed. In Low mode, the blower runs normally (slow).

Only in the cool of the evenings do the temperature wheel and Auto settings moderate blower speed.

I am correct in guessing the climate control system is attempting to cool the car by running the blower full blast, but the vacuum element, or the line to it, is apparently leaking and needs to be replaced -- a job the requires dismantling the dash?

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black