blowin hot air

Hi, Could someone give me a logical, systematic way to trouble shoot my A/C system on a 97 ks suburban. It is blowing hot. The coolant pressure was a bit low, so I bought a can of the coolant and added enough-- 1 can-- to bring the pressure to within normal range. That didn't help. Can someone tell me how to proceed. Thanks, Rob

One other thing. I noticed that the high side pressure valve hisses when I unscrewed the cap with the a/c on. Is that normal? (I am assuming the high side pressure is this one on the left when facing the engine, correct?)