The lines, in addition to swelling shut, get soft and can "balloon" under pressure.

As you have found, replacing brake components one at a time is wasted effort (unless one bit has obviously been damaged). By hard experience, on any car over 15 years old (kinda the "break" point for me, lol), I automatically replace all the flex-lines and give the cylinders/calipers a very hard look. Usually, I end up replacing the wheel cylinders and 50/50 on the MC/calipers.

Flush all the old fluid out. Go get a quart of DOT-4 and use it all.

Oh, that pesky booster! Check it critically as they tend to accumulate fluid when they fail (the "mystery" leak that can't be found). A dis-connected booster (from the manifold) is a suspicious sign of a failed booster. You shouldn't really "need" it but you will have to use more leg effort.

Going twice around sounds like a good idea but first ensure that you have decent rear lining left and that they are adjusted (if you got rear drums).

Very good place to spend first effort/dollars!

--
Mike!