I don't have the answers, but I know you can get cans of R-134a that come with a hose so you don't need anything else to use it. I don't know if it's possible to overfill it with the amount of pressure a can makes. In the past I saw cans with oil added to them. It's supposed to lube the compressor or valve or something. I was hoping someone else would have chimed in by now. My truck is 10 years older than your car and I might need to know this some day.

 

After I took the gauges off I noticed one of the schrader valves was hissing, and now the system is down to 0 psi. So I believe a faulty valve has been the culprit all along.

Now that the system is down I'm going to replace both valves, but my regular schrader valve removal tool won't fit down on the valve. The stem of the valve is too long and the tool won't go down around it to twist it out. I need a tool with prongs longer to fit down into the recessed sides of the valve. Does this '11 Crown Vic have schrader valves or a different kind of valve?

Also, when I go to fill the system with refrigerant, do I need to add any PEG oil or just add refrigerant up to the specified capacity?