AC system questions?

I have a 98 Expedition and the rear evaporator coil has a leak where the hoses connect. I assume there is an O-ring in this connection that has failed. I have pinpointed it to this spot because when I was running the AC, you could hear a pulsing hissing sound coming from the area. And when I pulled of the access panel, one of the two hoses coming from the front has brown oily stuff coming out of the connection.
I am not too familiar with AC systems, so I was hoping someone could help me out.
Can I just replace the O-ring and recharge the system, or do I have to get a vacuum pulled on it? How do I know if moisture is in the system?
When I push the low side schrader valve (the one where you put in refrigerant), it doesn't have much pressure at all if any. So is it safe to just disconnect the hose without having to worry about a bunch of pressure in the line?
Any help would be gratefull.

Forget what I said about no pressure, I actually just checked the pressure by pushing in the low side valve and it does have quite a bit of pressure left in it.
Any help?