The master is 50 bucks after core, if it comes off replace it.
I have a booster flake out on me, squishy pedal then mega hard. Almost had no brakes. Turns out the booster had a tear in it, so it would boost until the tear opened and dump all vacuum. At that point it felt like the floor but it really wasn't.
So I replaced both of them, and the soft lines. Massive improvement, feels like the truck is new.