If you are going through the cost and trouble of putting rear disc on your car, the MC is not a good place to cut cost.
http://www.mpbrakes.com/technical-support/troubleshooting-disc-01.cfm
When you add disc brakes to the rear you must use a true 4 wheel disc master cylinder. The pressure and volume requirements to the rear are much greater than that required by drums, disc brakes require more pressure and volume. A four wheel disc master will have a longer stroke and will provide more pressure to the rear disc brakes.
