^ this is correct. There is a low pressure cutoff so the compressor won't run on a dry system and self destruct. The lubricant is carried by the refrigerant so it protects itself by cutting off. To get things started you need to build enough pressure from the first can. I've never had an issue doing this. If it's cold outside you can put the can in a warm bath of water to help it along.

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2003 X5 3.0i MT, White, 161k