1994 Chevy S-10 warm air blowing

I have a 1994 Chevy S10 that had an AC compressor replaced about three summers ago due to worn bearings (making lots of noise). At the time the compressor was replaced the air was nice and cold meaning there had to be refrigerant in it (last time it was charged was summer 2001). About a week after I got the truck back I noticed the air was not as cool as it was the first day and the cold air was less and less. I took the truck back to the mechanics (Pep Boys) and told them the system was blowing warm air. They checked it and told me it was leaking and subsequently "repaired" it. About a week or so after the second trip to the garage it was blowing warm air again. Since it was near the end of summer I just skipped going back to Pep Boys for a THIRD time, saving that trip for the next summer. Well, they told me again that it was a leak in the system. Is this really possible? ANOTHER LEAK? I thought they were supposed to check these things before returning it to the customer.

Now understand that I really don't want to go back to Pep Boys so is it possible that the leak might be fixed with another can of refrigerant with sealant? Or just a can of sealant? If it does require some wrenches (replacing hoses or valves where would be the likely spots. Where should my focus lie?