First, watch what you're welding. Are some of those AC tubes aluminum? Or even plating can mess up your plans to weld it. You might end up just replacing the tubing.

Then, since it's been open for a while, your receiver/dryer has probably sucked up enough moisture from the air so it's shot. Get a new one.

After you fix all that, you HAVE TO pump the air out of the system. Running the compressor isn't going to do that.