I had this same situation on my El Camino. It would hold a vacuum but then leak like a sieve when the least little bit of fresh refrigerant was added. My leak was at a connector crimp and was easy to see with compressor oil residue and UV dye at the crimp.
Yes, I was surprised that it was still that low of a price but its roughly a 100 percent increase in price than one year ago at Walmart - $4.88. Still, I had two cans before buying these and it would be irresponsible to blow through this many cans without trying to find and repair the ...
It is probably low on refrigerant. The best advice around here would be to vacuum the system down and recover the refrigerant and then add back in the proper amount by weight. I'd probably add in some UV dye to check for leaks and maybe add some compressor oil back in.
Cold air does not hold as much moisture as warm air so the air sheds its water and becomes dry. With the AC set to defrost, the chilled AC air is dryer than the moisture on the windshield glass and the blowing air basically pulls the moisture off the glass as it blows over.
I thought I'd give this post a decent interval of a week before posting my answer - time to let the site owner Tim to respond. I got the one oddball connector adapter that I needed for my LS swapped El Camino from Nostalgic Air: