Derp, the title should say "compressor" and not "compression." Hopefully an easy question in the last paragraph for anyone with a II and functional factory A/C if you don't feel like reading the whole story. I just got my 78's A/C buttoned up (since I was changing out the heater core which was a monumental task, I figured I'd redo all the o-rings, etc. and convert to R134a). Got everything going, refilled the system, and experienced the first nice cool air the car probably blew in 25 years. No signs of leaks or anything from the hard parts (used some florescent pag-oil), no hissing, etc., so I was pretty excited. About an hour in, I found myself bailing out of the car and fleeing the white cloud of R134a that catastrophically found its way out of the low-pressure line (which had apparently rubbed against the alternator at some point over the last 35 years and weakened the line in a location I hadn't seen). (Sorry to mother nature, it wasn't intentional and it least it wasn't R12). $40 and a trip to Napa for a custom hose assembly and I'm ready to refill. So this all has very little to do with my question (other than to give you a little amusement maybe picturing someone fleeing a smoking II for their life, I'm sure the neighbors had a good laugh before the noxious cloud overwhelmed them). Before I refill again, one thing I noticed was that my compressor runs all the time (clutch engages) when the hvac control is on A/C. It doesn't care whether the system is charged or isn't charged, it's just on when I tell it to go on. It doesn't cycle off either, which is how I'm used to Ford A/C working (usually with a pressure switch in the clutch circuit). So as I mentioned I had the whole A/C apart and at no time came across any electrical sensors or connections anyplace (other than the compressor clutch and that ambient-air sensor in front which I have no idea what it's supposed to do). Is this how the Mustang II factory air conditioning compressor is supposed to work? Always on when selected? Or do I have a shorted or bypassed switch somewhere that I haven't come across?