Drum brake master cylinders need something called "residual pressure" in the lines to prevent air from finding its way past the wheel cylinder cups. Any residual pressure at all will cause a disc brake to drag, so a M/C for a disc system will not have the pressure check valves the drum system needs. Also, a M/C for a disc brake setup will have a much larger reservoir on the disc brake end of the system. The diameter of the master cylinder piston will almost certainly be different than the 1/2 ton cylinder, also. This affects the volume of fluid that's displaced as the pedal is pushed, and the hydraulic advantage of the M/C over the wheel cylinders will also be totally incorrect. You're betting your life on your brakes every time you push the pedal, plus the lives of everyone else on the road near you. It's much better to use brand new, correct parts for the job at hand where brake work is concerned.
Jerry
