With the autozone kits you have no way to actually measure the amount of freon in the system and how much you are adding. r134a systems are very particular about the amount in them to work most efficiently. Its not even as simple as reading a sight glass on a system with a leak.. you get air in the system etc. To properly fill a system you need to:

#Evacuate and store any refrigerant left in the system
#Vacuum system for atleast 15 mins, then i usually let sit for 10+ mins to watch if the system loses vaccuum (aka leak)
#Inject dye + oil
#recharge system with proper amount of freon

If your system has been open for a while too due to a leak you will probably want to replace your dryer since its most likely saturated with moisture.