Ok, definitely sounds like a refrigerant leak. To repair this correctly you need to recover the refrigerant and pull the system down into a vacuum. Pressurize the system with nitrogen and try to find the leak, repair the leak, replace the dryer or install a new one, then recharge the system. Most likely the refrigerator uses R-12 which you need a license to buy, if you can even find any. There are no drop in replacements for the R-12 either. A different refrigerant will not be compatible with the compressor oil. The other problem is you have air in the refrigeration system. If refrigerant has leaked out, air has leaked in. You can try your can of sealant if you'd like but I really don't think it will work. You would want to tap into the large line that goes from the compressor up to the freezer box. That's the low pressure suction line.