Master cylinders have O-ring seals in them. As they wear against the iron or aluminum surface of the bore, they wear out. Depending on how the pressure is applied, they will "catch" and seal, or sometimes they will not seal and fall toward the floor.
Ever have one of those old-school bike pumps? As the seal wears out, sometimes it catches and pushes pressure, and sometimes you almost fall on your face because it doesn't catch pressure.