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First things first I must warn You at minimum You need a CFC liscense to purchase refrigerant. That said I also want to point out that most freezers produced after 1990 (some ealier) have a fraction of the charge of refrigerant in them, if a freezer needs charged that means it has a leak. Even with a lowside leak You will be lucky to get a week out of a unit thats recharged and not repaired.
Refrigeration 101, basic operation : First thing to understand is that the term "COLD" is relative meaning You can feel cold, or that is cold to the touch but there is technicaly no such thing as cold as there is heat available to be obtain to well under -400°F. Now knowing this a refrigeration system merely moves heat from an unwated location to a location of insignificance ie: inside a freezer to outside. this is accomplished by a pump called the comressor, and a series of tubes and coils refered to as a sealed system (notice the word sealed meaning nothing gets in nothing gets out unless someone either puts it there or You have a failed system). There are two coils properly named the evaporator and the condensor, You have heard the term "Change of State" for example an ice cube melting is changing state from a solid to a liquid its state of existance is changing due to its enviroment. In a sealed system they take advantage of such a change because the most heat is absorbed or released during changes of state.
In a system this change is from a liquid to a gas then back to a liquid again.
In Your freezer the compressor takes this gas and comresses it in the condensor untill it condenses forcing it to change state the heat that was absorbed to allow it to be a gas is now forced out as it changes to a liquid. The liquid then passes through a small tube called the capillary tube untill it reaches the inside of the freezer and exits into the evaporator where the liquid refrigerant goes from a high pressure to a low pressure enviroment forcing it to change to a gas this change forces the absorbtion of heat that is carried to the compressor to be recompressed and forced to give off this heat outside the freezer, then through a filter and back out the capillary tube to do it all again.
As far as charging the unit goes:
Assuming You repaired the leak the easiest way is to solder a tap or valve to both sides of the compressor the high side and the low side (high and low pressure) start by recovering any refrigerant that could be left in the system, and to help remove any moisture. You would then read the model tag to verify what refrigerant is in Your freezer and the charge in ounces. The easiest way to clean out the system is to apply a sweep charge which is 80% of the recommended final charge on the tag and allow the unit to run for 15 minutes, recover the sweep charge and measure in the proper final charge seal up your taps and Your finished.
Now for the bad news if the freezer is using 134a the oil in the compressor is no good if the unit has had a leak, this oil absorbs moisture and turns to gel starving the compressor of lubrication, and in return causes a more severe failure one that usualy results in flushing the coils and lines.
Hope this is informative enough to help with Your decisions.
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