I've had slow hard to find leaks too. There are a lot of O-rings in that system and sometimes they hold up for 20yrs, sometimes only a few. So what I do is buy the sealant. I used this one last, worked great; amazon.com/dp/B086KYJSLG I used this to put it in; amazon.com/dp/B07FCFWXNQ along with some freon of course, which I bought a six pack or so of.
I think I spent like $70 or so all together and have plenty of freon left over.
That hose assy has a check valve in it so you don't need to have a can on it to read your pressure. Just use it like a a tire pressure gauge, pop it on, check psi. Mine likes to be on the higher end, I think it 45 psi? Likely due to it being hot where I live which is currently >100.
If it still leaks then you can use a UV light to look for it since the sealer has UV die in it. I have a UV light I got on ebay; ebay.com/itm/361959635949