One comment and a two suggestions -
It is very possible that the compressor is siezed or not getting power. Check this before you try locating the leak.
1. If you have a Merchant's Auto in your area, they will do a "diagnosis" for $40 plus a pound of freon. Take the splash pan off so that the compressor is viewable.
2. Read my post "Cold AC but No Money" for what it took me to get cold AC in a car that I just bought.
Good luck - there are some truly grim outcomes to this problem.