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