LP Connection
Port nearest the firewall is the LP port and that that's the one you charge to w/ compressor running. Charge to the HP port and you're asking for a bad day because you can over pressurize the can and then have it explode. Always charge gas to the system (top of the can up). Charging liquid (can upside down) can damage the compressor suction valves. There are exceptions, if you are using blended refrigerants (like Hot Shot or Freeze-12) then you carefully and slowly charge as liquid to maintain the proper ratio of the blend. HP & LP ports are different sizes so hooking up to the wrong port shouldn't be a problem. HP port is for pressure readings and system evacuation