You don't need to evacuate to check for leaks, just put some UV dye in the existing charge since the compressor is still kicking in. Then use a UV light after a day or so to look for leaks. If you find no leaks at a connection, look at the condensation drain water with the UV lamp. If there is UV in the water, the problem is the evaporator.
High side connection is somewhere in line around filter drier. Low side is somewhere in line between compressor and evaporator.
It would be best to do some reading for safety sake. Without a/c understanding you can hurt yourself or your car.