Could also be a bad check valve leaking all the vac out of the AC or door reservoir, too -- sounds just like a ballon venting out a stretched mouth.

The check valve on the vac pump could be going bad, too, same result.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!