My view is a little more optimistic as it's claimed 10% of refrigerant permeates from car A/C system per year. My Mondeo is 7 years old therefore I am assuming it has slowly lost 70% of gas. I would think with that amount of loss the LP switch would operate and prevent clutch/compressor from being energized. Bearing in mind Mr Ford charged me £8.71 for a small selection of shims (which in reality are cheap washers) I am happy to have A/C fully restored today for 49 quid by Kwik-fit.