Vaccum Reservoir removal/replace.

I'm expriencing with an anomaly with in the AC system of my 2001 SLK320...."hissing" noise from the passenger's leg area. As long as the AC is on, it's fine. As soon as I turn the AC off....it hisses. The switch-over valve is a new replacement and working. With the glove box removed, there's a unit (a small black box with orange color rubber in between 2 hard plastic casing pressed together) directly above the switch-over valve. One of the 2 valves from the switch-over valve connected directly to this unit. The hissing noise comes from this unit and not the switch-over valve. I can get to this vaccum resevoir, but do not know how to safely remove it without breaking anything else. Does anyone have any ideas or information of how to do this tidious job???.....Please help. Many thanks.