My understanding is.
Halfords or anywhere that is doing a re-gas for just a few ponds will be using one of these automated systems.
The operator appears to attach a couple of pipes and dialsl up the car/model.
The machine sucks out the existing contents and by law recovers it.
Applies a vacuum for a specified time and if all is OK ( not leaking i.e sucking air in ) then fills the system with the correct amount of gas and hopefully lubricating oil.
If the vacuum test fails the process stops there and your car has NO gas in it.
An A/C guy can suck out the gas and then for example fit a new( not expensive) drier in the system before re-gassing it..
He can also check the compressor, it clutch etc are all working correctly.
If there is a leak then a dye test may show where the problem is.
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