Take it to a HVAC specialist.
I don't mean to be rude, but not understanding the basics of the AC system, risks, your health, safety and operation of your equipment. It's not rocket science, but you can get hurt as well as your equipment not knowing the proper way to charge a system, and if done wrong, will cost more than having a professional do it. If you insist on learning and doing it yourself, then spend the time reading how tos and watching YouTube vids. The information is out there, just make sure you are confident before you tackle it safely.

__________________
Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)

x2

You will hear a hissing sound if it is vacuum ( air rushing in ) or if pressurized ( freon rushing out ).

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.