I have a 2001 Sentra 1.8L. When I turn the air conditioner on I hear a hissing sound, like a vacuum leak, and the system will not blow cold air. It will only blow ambient temperature air. I have tried adding refrigerant, but the system barely takes it in. The air will get cooler, but not cold like it used to be. So, my question is, do I have a controls problem or a system problem? Vacuum leak not telling the system to function properly, or something else wrong with the mechanical system? Thanks!