I don't know how large the evaporator is.. it may be pretty small... so instead of messing with the refrigerant... just put it under water the way they check for leaks in tires at the station. A drop of Joy detergent will make tiny leaks show up dramatically with bubbles. You can probably do this in a regular dishpan... if necessary diagonally...
I offer this because many people don't have electronic leak detectors available.. ( I don't ).