Matt
I had my R12 topped up recently, everything going fine. So I have not made the changeover, but I can tell you what I know from the technician:
They purge your system, by connecting it to a vacuum. This takes all the old refrigerant out. They have to change the condenser, because the connector for R134 is a different size. This costs a good few bucks. Also, they tend to want to change seals, but this is not major costly, and I think the job can be done without putting in new seals.
Then they recharge using the new gas. The trick to get it right seems to be putting in the correct poundage, so the differential between the high pressure side and the low pressure side is the correct value. They may also add a dye to the new gas, which will show if you have a leak.
The system turns itself off if there is insufficient gas inside to protect the compressor. This is because the gas contains lubricant, without lube, the compressor burns up. So no gas = no cooling, and don't turn on the AC manually if you are low on gas.
Lastly, the clicking you hear would indicate maybe a sticking door or solenoid, some kind of problem possibly not related to low gas pressure. Search for the AC diagnostic routine on the forum. It is probably similar for a Legacy, and worth a try to highlight what electrical fault you may have, before you go to the expense of recharging with whatever gas.
Best of luck
Joe
