Wish I could help, but I sometimes find myself in the same boat. My source from whom I could borrow the sensitive sniffer (which helped about 50% of the time) moved away. My last try with the UV dye to find a slow leak met with no result (other than me longing for my youth and old teenage bedroom with the Zonk and SuperPeace posters).

I ended up taking the percentage bet, and throwing a [thankfully on hand, good used] compressor at the problem. It fixed it.

So I guess my constructive contribution here is to