Usually, you have to buy a new hose to seal a flare....although some boxes have removable fittings in the box. The flare fittings seal by an contact line just like your engine valves and your brake fittings. One side is cut with an angle...say 45 deg and the other side is made to perhaps 46 deg. When the two are mated, a line contact is established. At this line contact, the pressure is high (since it is a thin line) and the material is deformed slightly to make the seal. If you ever look closely at your brake fittings, you will see a ring or line around the cone on the fitting....this is the contact line. When the fitting is overtightened, the fitting is deformed and leaks since the material is permanently deformed.....this is why the fittings do not require excessive tightening.

Hoses are cheap. You can take your hoses to a hydraulic supply and they can make them for you if they have the particular fitting. Also, the auto supply carries replacement hoses.