I am a professional hvac tech.if you need refrigerant then there is a leak. First make sure the coils are clean if they sre not ckean you will not get true pressure tempitures. Dirty coils will cause a lot of problems. Use good gages the cheap gauges are not very accurate. Check for leajs if it is low on freon. Common easy to fix leaks are Shrader valves but the tool is 49 dollars. A cheap one wull work.. if it is low use soapy water wher there are welds it will make bubbles where the leak is. The new refrigerants are a much higher pressure than r22 or r12. Going to 134a you will not be happy. Imho if it is more than cleaning coils replace it. Why you ask. The old refrigerants are getting expencive. A newer system will be more efficient. Make sure the fan blades are clean.