Putting gauges on it would be a good place to start.
Maybe Autozone has them in their rental program?
The easiest way to find a leak is to look for oil. When refrigerant leaks oil leaks out too. On a pressurized system you can use soapy water. Or some kind of leak detector.
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You will also need new o-rings, nylog, a vacuum pump, refrigerant and the proper type of oil. Not to mention a new receiver/dryer and any other parts that may be needed to fix the leak.
Danny