OK the problem with Air Con is that the compressor that pumps your refrigerant gas around the system will not start unless the system is charged. This is a fail safe, to stop the compressor running when there is no gas in the system normally because of a leek. NO GAS MEANS NO OIL and then your compressor will seize.
Its one massive viscous circle, i work in commercial Air Con and the car companies can and will rip you off because the general public have no idea how the system works. All you can try and do is find a reliable person to sort it out which i know is very difficult.
Andy
January 2008