R-12 has been the long defacto standard for MVAC where the manifold gauge set used a 7/16-18 thread and the ports were the same so essentially either side of the manifold gauges would fit. The first change came about when GM was concerned about techs confusing the low side with the high side, so the high side was made smaller. While I have yet to see a red hose with this smaller connector, adapters were readily available. But you still needed sense to connect that adapter to the red hose and connect it to the high side. In later years, GM came out with a new high side port that required yet another adapter, claimed the existing smaller high port tended to leak for the reason for the new adapter.
When R-134a came along, again assumptions were made that techs can read, so the manifold hoses besides other configuration changes has a 1/2-16 thread. But if you still wanted to use your R-12 stuff, no problem could by an R-12 to an R-134a adapter, needed these for your vacuum pump, can tap, or tank fitting. But if you purchased R-134a port adapters still could use your old R-12 vacuum pump and can tap, but needed an adapter if you purchased R-134a in tanks.
One advantage of having two R-12 manifold gauges, is that you can use the blue hose of the first to connect to your vacuum pump and connected the read to your refrigerant source, and connect the yellow of the first to the yellow of the second set of gauges with its blue connected to the low side and red to the high side. In this way, you can open all the manifold valves but keep the refrigerant valve closed and draw a nice long vacuum clear up to the refrigerant valve. Then close off the valve to the vacuum pump and open the refrigerant source valve to get an initial charge into the system. Close off the high side valve and charge only on the low side with the engine running at 1,500 rpm, doors open, AC on, and blower at maximum to reach your proper pressures based on relative humidity and ambient temperatures. With a tank, no need to purge and change cans. But if using those PITA cans, can remove the first set of gauges so you have far less to purge. At this time, you have a high positive pressure in the system where a little refrigerant will leak out, but air won't leak in. The advantage of having two gauge sets, as you can switch from vacuuming to charging without changing hoses.
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