Service Port HVAC: Description and Operation

Fig 4 A/C Service Ports:

The refrigerant system service ports are used to charge,recover/recycle, evacuate, and test the air conditioning refrigerant system. Unique service port fitting sizes are used on the R-134a system, to ensure that the refrigerant system is not accidentally contaminated by the use of the wrong refrigerant (R-12), or refrigerant system service equipment.

After servicing the refrigerant system, always reinstall the service port caps.