Homelessduck,
At a minimum -
You should actually draw down the system with vacuum for 30 minutes (Eliminates any moisture in the system.), shut the valve running to the hose to the vacuum pump, see if it holds for 60 minutes, then you can recharge if it does. Your auto part store may have what you need for loan or rent. There are some inexpensive kits (Vacuum pump and Gauges) out there at HF, eBay, and Amazon.
If you do have a R12 system. There are also R12 to R134 conversion connectors so you can use a R134 gauge. Also there are stickers available that state you have converted from R12 to R134 so folks know that in the future if you are not in the picture.
Better -
Replace the A/C dryer. (Desiccant usually fills up to capacity when the system is left open for a while.)
Replace the orifice tub. (If clogged, semi-clogged, or going to R134 it should be changed to the appropriate one.) Another member said he used a red with good results. They make special pullers to get yours out. (Amazon has a kit by Robin Air for about $35.) If the tip breaks off with pliers its a bit of a ordeal to get them out.
Replace any A/C O-rings on fittings you opened.
If you suspect you have any leaks add some dye. Then you can see the leaks with UV light.
Only use refrigerant, not one with stop leak or oil. (You do not want to mix the older oil with the newer stuff. Stop leak will clog stuff up. If you need oil add only ester oil.)
There was a thread on this not too long ago that is worth a read.
https://www.oilburners.net/threads/recommendations-for-ac-coolant.89698/
I have heard that R-12 hoses can allow R-134 to pass through the rubber part of the line. (Slowly leak out over time.) Curious if anyone has noticed after going to R-134 each year they are low and have to add more R-134. Getting ready to replace my A/C compressor and thought good time to change the rubber lines if so.
Stay cool and let us know how it turns out!