They have special oil which disolves in the gas to lubricate the compressor - if the gas escapes the oil can too. I have found that when the car units break it is often due to no oil having been rplaced.
An a/c engineer (or garage) can check the pressure and add gas if it is low. One of these engineers told me (no idea if its true) that they often leak from lack of use due to the compressor seals sticking to the shafts - you are supposed to use regularly - even in winter 
-------------------- All men dream, but not equally.
Those who dream by night, in the dusty recesses of their minds,
wake in the day to find that it was vanity.
But the dreamers of the day are dangerous men,
for they may act their dreams with open eyes, to make it possible.
- T.E. Lawrence -
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