Yeah, I saw the sight glass but is is like one of those Delco battery sights. Just a little dot. Anyway, CSS with it to tell you the truth. If you do a little figuring, a totally empty system will take a lot of stuff, Gotta figure the compressor, lines, canisters, a/c heat exchangers, etc. It is a liquid, so if you use 6 cans it is only 72 ounces - 2.25 quarts - not much. After 13 years of life, the stuff will leak out somewhere over time, everything else does. A true leak through a fitting or the like will be very obvious even without the special dye some use - the oil in the 134a leaves a distinctive oil stain. Anyway, that worked for me. Sure cheaper than a shop doing.