You can overpressurize the system, but you have to work at it!! I did it once.

It was a cold and stormy night, the next day was a non-stop from the Twin Cities to Columbus, Ohio. The temps here were 60-65F, Columbus was 80-90F, so I grabbed 2 cans of gas, ran the engine and dumped in the entire 2 cans! AC was cold!

Next morning, after loading the car, I started the drive down the street with the heat on and something making a strange noise in the engine compartment. The noise stopped after 500ft, so I hit the highway and didn't look back.

Climate control worked fine for 13 hours, and I parked at the motel. Temp was 75F. Next morning, I drove to my daughter's dorm, parked in the parking lot and unloaded. A couple of hours later, the shade temp was about 88F.

I started the car, pulled out of the lot and BANG!!! from under the hood. I immediately pulled over and popped the hood. There was greenish fluid on the front side of the engine and a good sized puddle on the ground. I rolled down hill to a local repair shop and asked them to take a look. They couldn't find anything wrong, I thought it might have been coolant, but they said they couldn't see anything leaking.

I called the local dealer and drove the 10 miles there. After a bit of searching, they wiped down the smooth metal parts and concluded the pressure valve popped and I lost a bunch of R134a. The AC still worked. It worked all the way home!!

So don't worry about too much pressure, about 600psi will pop the valve. But adding too much R134a will cause the AC to be less efficient. To little and the clutch plate will wear out due to all of the cycling.
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Klausc, a Volvoholic