71 AC question?

[myyamr7]

I'm working on getting my AC system working again after having the truck apart. I bought it over the winter, so I never bothered to try and turn it on. The system at the time, was complete and had a charge.
Just because it was apart, I replaced all the lines and hoses, compressor, dryer and a few other minor parts.
I have an appointment next week to get it charged, but when I was hooking everything up, the compressor kicked on? There is no charge in the system at all. After a little messing around under the dash, I found it would only kick on when I turn the fan on? I figured it was the AC fan relay, so I replaced that with a new one. Same thing still happens.
I hate to keep tossing parts at it playing the guessing game, does anyone have any idea what could cause this? AC relay itself? Dash Control switches? or does it just need the charge for the system to work correctly all together?

Thanks in advance for any help