Well .... i have seen this brake lines were perforated... real rust micro holes.
Had to hunt and watch while someone pumped brakes .
Could see a little fluid being pushed out then sucked in through the rust.
Never did drip any fluid .. but was sucking air in.
About drove us crazy till we found it. Replaced lines good to go.
P.S ADVANCED PARTS ARE USELESS AS TEATS ON A BOAR HOG LOL.