Less common but possible, is a ballooning rubber brake line. The result of the line failing internally.
Being that you are looking at rebuilding the system, I would also replace the brake hoses, especially if they are of unknown age.
The pressure is going somewhere and without a leak that can only be a couple places.
It's bleeding back on itself (MC failure), expanding the wrong thing (ballooning a line), not expanding something enough (incorrect/worn parts and/or adjustment).
You could also have contaminated shoes.
