Automotive airconditioning systems are not a sealed system in the same sense as the system in your refrigerator, for example. The very nature of the belt driven automotive AC compressor requires a shaft seal to prevent the uncontrolled loss of refrigerant where the shaft exits the compressor housing. The shaft seal does NOT totally prevent refrigerant from leaking, as some acceptable amount will leak past the shaft seal. That's one reason home AC and refrigeration systems use hermetically sealed compressors, where the motor and compressor are sealed in one housing, and there are no shaft seals required. In addition, car AC systems require rubber hoses at certain points, to allow for the movement of the engine relative to the rest of the car, and the rubber compounds will allow the very slow migration of refrigerant molecules through it over time. One reason the automakers couldn't simply switch to R-22 when R-12 became illegal to utilize was that R-22's molecular size is much smaller than R-12's and would leak unacceptably fast through the rubber refrigerant hoses. So some degree of leakage is natural with automotive AC systems, but if you have to recharge more often than every 5-10 years, then the leakage has grown to unacceptable levels. And yes, Ford seems to insist on using O-rings at its connections, which are notorious for leaking after a few years.
