What he may be referring to is you can only get 10 oz of freon out of the 12 oz can. I don't know a lot about ac systems, so this is just an educated guess. The pressure inside the vessel can't be reduced lower than the pressure of the system being charged (unless the system draws a vacuum on the can), there must always be some material left in the can. If the compressor "sucks" freon out of the can, then you should be able to remove all the contents. But if you are relying only on the pressure inside the can to deliver the contents, you won't be able to remove everything.

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