I have 2 decent cylinders of R134a refrigerant and what appears to be the correct adaptor &c but can't introduce anything into the a.c. system through the low pressure side. The compressor doesn't run now so suspect the refrigerant is empty. Shouldn't I be able to introduce enough coolant to just get the compressor running, naturally I had the circulation switch on as well and the blower set to 4.
Typically I didn't try it until this recent hot weather, perhaps I'll rember in future!

You need to apply vacuum to the system for at least 30 minutes to remove moisture and to check for leaks in the system.

Then when you release the gas to the system it is sucked in by the vacuum and also the gas needs to be gently heated. Additionally it is filled via both ports as you need to “vent” the system. It is illegal to release ac gas to the atmosphere.

The system won’t suck it in unless the compressor is running. Also you need to make sure you don’t over or under fill the system so you need to measure the amount put in.

If you don’t have an ac machine or equipment then I’d recommend taking it to someone who has.

In addition to td4van's information. If you have over pressurised the system, the trinary switch will shut the AC compressor off again. Messing with the AC system is a professional job, that requires professional equipment to do safely and environmentally.

I am taking it to my usual service place, it's certainly not overfilled and they have a system to detect obvious leaks.

It's been "sucked" and dried, there was 90gms left but clearly not enough and, having omitted this wet spring to run it at all, it's working better than ever. Mea Culpa!