Your drawing is correct. During normal operation the stem is backed out and seated to seal off the service port.
When servicing the system, you need to turn them both in but not all the way. That allows you to evacuate and charge it.
In the event you need to isolate the compressor, and not lose all of your refrigerant, you turn them all the way in until they seat. You can then remove the fitting from the compressor, or open the oil plug to check the oil level and add more oil if needed.
bigwow.It's the same on my 88 Renix.
yeah 100%.No worries. Just make sure the valves on the manifold gauge are closed before you open the service valves.
0 high side pressure.
30 low side