I know we have some chemists on the list.

I guess I admit to being a chemist but I'm really rusty on this kind of
stuff. First, pure hydrochloric acid will not dissolve copper metal, but
hcl with oxygen dissolved in it will, so that is one difference between
fresh acid and "used" stuff. The used stuff most likely does have copper
ions dissolved in it. Then, since zinc is more electrochemically active
than copper, when you put the copper ions in contact with the zinc metal the
zinc is oxidized and the copper will plate out. Now, at the same time the
hcl is chewing up the zinc as fast as it can so it comes down to how "used
up" the acid is. If the hcl is mostly consumed so it doesn't react with the
zinc very fast, and if the solution has picked up lots of dissolved air
(oxygen) in use and then used that to dissolve lots of copper along the way,
that copper will have time to plate out on the zinc before the remaining hcl
eats away the zinc. If you took fresh hcl and just added pure zinc until
the acid was mostly used up, it should not plate copper out if you then put
it on brass. This is all my take on this without doing any research to back
it up, ymmv, it's worth what you paid for it, etc ;-).

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Regards,
Carl Ijames