I have always used the cans of "freon" that have the sealer stuff in it.

 

The expansion valve is internal to the a/c unit inside the car should this fail the dash needs to come out. Not what you want to have happen.

 

if it fails then the only AC I will have is the TOP DOWN.

 

One thing to watch is that A/C can leak THROUGH the actual hose itself. I tried tracking down my XK8 leak and couldn't find it anywhere, not with dye, compressed air, nothing. It turns out that the rubber low-pressure hose from the low-pressure port at the firewall that goes to the compressor was oozing mostly from the bottom. A mechanic buddy of mine casually wiped the bottom of the hose with a white cloth and showed me the pale green dye with a UV light in a darkened room. It was also covered in the compressor oil, which was one of the reasons he checked there first. I didn't know freon could leak THROUGH a hose.