It's been years but here is what my chemically altered, feeble mind remembers
The correct way is to replaced the proportioning they are different for drum to disc also the master cylinders are different. The calipers of course use more fluid so the reservoir for the front is larger, What I'm not sure of is if the M/C bore is larger. Regardless replace both. If this is a trail rig only you may want to see how it behaves on the trail before laying out the coin for those pieces.