Unfortunately it is not that simple.
If what was inside leaked out; you will need to find the leak first.
Next is if the sysetm has been exposed to Air a part called the Dryer gets plugged up due to internal rust and needs to be replaced.
The above is what little I know about it; for more info go up to the upper rignt hand page to the "Resourses" buttong.
DIY Articles, and DIY Links will shoe up.
Look through both of those for A/C repairs.

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84 300D, 82 Volvo 244Gl Diesel