Look guys it's really hard to say what the cause of it could be, it could even be your body's sensitivity to pure ethanol.
But as I've always said, and this is not directed at any one person but is a good reminder for all even if in this case I'm wrong.
Using scrap metal yards to buy cheap pipe is stupid, you do not know the origins of the pipe and neither does the scrappy.
It could contain anything from medical gas residue to sewerage and other things too, copper will suck these compounds in and it's very hard to get it back out no matter how much you clean it.
I'm not saying this to start a debate, or some kind of scare campaign, I'm just repeating what I've always thought and said on the matter.
FFS you're building something you're going to be using for many years saving money on booze, and this myth that copper pipe is expensive is bullshit when you can pick up a meter of 4" for under $100 :teasing-blah: :teasing-blah: :teasing-blah:
Now this may not be the problem at all, however if new pipe was used these issues are automatically eliminated.
Buy new, be safe & set your mind at ease.... it's going to last you forever & you dont want the worry of "what if" do you ?