Slightly open the valves, cracked open not much, and purge the hoses until you hear the refrigerant hissing out, not long, a few seconds.

I'm not sure about the half-hour time to empty the can. If / when I introduce refrigerant on my system, it has always been in Florida, when very warm outside, and the box also warm. And takes way less than half-hour. I introduce very slowly, wait several minutes, watch for foam / froth / bubbles, and keep doing this until the Glass clears.

I'm not an expert, but I would add a bit more refrigerant … until the Glass clears. My system takes (2) 12oz cans when empty. So, I wouldn't add much.

I would look for leaks. Check every connection, feel around for oily substance. You could also have a loose connection somewhere. You can mix 50/50 water and dishwashing soap, coat the fittings & connections with this and look for bubbles, if you see bubbles you probably have a leak there. Leaks tend to occur on the bottom of fittings and connections. If you have lines covered in insulation it could be there is corrosion and small leak. Doesn't sound like you have a massive leak. Tiny leaks are tough to find. If you can add a can and be fine for some time, a week or more, then I would keep watching the system and adding cans if need be until a pro can find the leak using soap or a electronic leak detector. A/C guys can do this. Again, easiest to find leaks with the system warm, pressures higher.

Sometimes if the system has not been run for some time, seals in the Compressor dry up and leak refrigerant, but once started up the oil in the system can lube the seals and no more leaking. So maybe you'll be fine … hope so !

Cheers, John