2000 GMC Sonoma A/C not cold anymore

I have a 2000 GMC Sonoma V6 4.3L. I got the truck at like 40,000 miles 7 years ago, A/C worked fine except it used a lot of gas and robbed engine power. So I used my a/c sparingly. After a while, I never used my a/c. At the most I turned it on maybe once a year for a few years. For the past 3-4 years, I would turn it on like once a year to see if it worked. I heard the clutch come on, but over the years it began to blow warmer air. This past week, I wanted to see if I needed just a recharge because I am going on a trip. I turned on the a/c and the clutch wouldn't engage now. I bought one of those recharge kits at and auto store, and my friend helped me out. Put a little bit in and the a/c clutch turned on, but it appears that I already had enough refrigerant. Do I need to evacuate the system now and start with new refrigerant? Because it doesn't blow cold air.